# Read a file
try:
f = open("demofile.txt")
f.close()
print("Accessed file OK")
except FileNotFoundError:
print("File does not exist")
finally:
print("All done.")
"""
Syntax:
open("filename", "mode")
Default mode values:
r = read
t = text
These are the same
f = open("demofile.txt")
f = open("demofile.txt", "rt")
Open Modes:
r = read
a = append = Open for appending, creates if file does not exist
w = write = Open for writing, creates if file does not exist
x = create = Creates the file for writing, returns error if already exists
Handling modes:
t = text
b = binary
"""
# Open, read, and display a file
f = open("c:\\temp\\demofile.txt", "r")
print(f.read())
f.close()
# Open and read 5 characters
f = open("demofile.txt", "r")
# "This "
print(f.read(5))
f.close()
# Open and read a line
f = open("demofile.txt", "r")
# "This is"
print(f.readline())
f.close()
# Open and read two lines
f = open("demofile.txt", "r")
# "This is"
print(f.readline())
print(f.readline())
f.close()
# Open and loop through each line of the file
f = open("demofile.txt", "r")
for x in f:
print(x)
f.close
# Append to a file
f = open("demofile2.txt", "a")
f.write("Now the file has more content!")
f.close
# Open and loop through each line of the file
f = open("demofile2.txt", "r")
for x in f:
print(x)
f.close
# Overwrite a file if it exists
f = open("demofile3.txt", "w")
f.write("Woops! I have deleted the content!")
f.close()
# Delete a file
import os
os.remove("demofile2.txt")
# If the file exists, delete it
if os.path.exists("demofile3.txt"):
os.remove("demofile3.txt")
else:
print("The file does not exist")
Sunday, July 3, 2022
Saturday, July 2, 2022
Python - String formatting
price = 49
txt = "The price is {} dollars"
# The price is 49 dollars
print(txt.format(price))
name = "Fred"
txt = "The price is {} dollars, {}."
# The price is 49 dollars, Fred.
print(txt.format(price, name))
txt = "The price is {:.2f} dollars"
#The price is 49.00 dollars
print(txt.format(price))
quantity = 3
itemno = 567
price = 49
myorder = "I want {0} pieces of item number {1} for {2:.2f} dollars."
# I want 3 pieces of item number 567 for 49.00 dollars.
print(myorder.format(quantity, itemno, price))
myorder = "I have a {carname}, it is a {model}."
# I have a Ford, it is a Mustang.
print(myorder.format(carname = "Ford", model = "Mustang"))
txt = "The price is {} dollars"
# The price is 49 dollars
print(txt.format(price))
name = "Fred"
txt = "The price is {} dollars, {}."
# The price is 49 dollars, Fred.
print(txt.format(price, name))
txt = "The price is {:.2f} dollars"
#The price is 49.00 dollars
print(txt.format(price))
quantity = 3
itemno = 567
price = 49
myorder = "I want {0} pieces of item number {1} for {2:.2f} dollars."
# I want 3 pieces of item number 567 for 49.00 dollars.
print(myorder.format(quantity, itemno, price))
myorder = "I have a {carname}, it is a {model}."
# I have a Ford, it is a Mustang.
print(myorder.format(carname = "Ford", model = "Mustang"))
Python - User input
# User input for a string
username = input("Enter username:")
print("Hello, " + username)
# Note that this only gets a string.
# It does not scan for key presses
Python - Error handling (try, except, else, finally)
x = "hello world"
try:
print(x)
# This will cause an error
#y = 3 / 0
except NameError:
# Catch a specific error type
print("Forgot to define x")
except:
# Catch all other errors
print("An exception occured")
else:
# Run this if there was no error
print("All is well")
finally:
# Do this regardless of error status
print("That's all folks")
# Throw an error on purpose
x = -1
if x < 0:
raise Exception("Sorry, no numbers below zero")
try:
print(x)
# This will cause an error
#y = 3 / 0
except NameError:
# Catch a specific error type
print("Forgot to define x")
except:
# Catch all other errors
print("An exception occured")
else:
# Run this if there was no error
print("All is well")
finally:
# Do this regardless of error status
print("That's all folks")
# Throw an error on purpose
x = -1
if x < 0:
raise Exception("Sorry, no numbers below zero")
Python - Regular Expressions
# Import the regular expression module
import re
txt = "The rain in Spain"
# This searches for:
# ^The = "The" at the beginning of the line (^)
# Spain$ = "Spain" at the end of the line ($)
x = re.search("^The.*Spain$", txt)
print(x)
# Pretty much any populated variable evaluates to True
# So if we got a match, that value is tucked into x
# Which means this will evaluate to True if we got a match
if bool(x) == True:
print("We got a match")
else:
print("We did not geta match")
# Split returns a list where the string has been split at each match
# So the next line returns all the words separated by a space:
# The, rain, in, Spain
words = re.split(" ", txt)
print(words)
# Sub replaces one or many matches with a string
new = re.sub("Spain", "England", txt)
# The rain in England
print(new)
# Review regular expression matching
# A quick review sheet is here:
# https://www.w3schools.com/python/python_regex.asp
import re
txt = "The rain in Spain"
# This searches for:
# ^The = "The" at the beginning of the line (^)
# Spain$ = "Spain" at the end of the line ($)
x = re.search("^The.*Spain$", txt)
print(x)
# Pretty much any populated variable evaluates to True
# So if we got a match, that value is tucked into x
# Which means this will evaluate to True if we got a match
if bool(x) == True:
print("We got a match")
else:
print("We did not geta match")
# Split returns a list where the string has been split at each match
# So the next line returns all the words separated by a space:
# The, rain, in, Spain
words = re.split(" ", txt)
print(words)
# Sub replaces one or many matches with a string
new = re.sub("Spain", "England", txt)
# The rain in England
print(new)
# Review regular expression matching
# A quick review sheet is here:
# https://www.w3schools.com/python/python_regex.asp
Python - Math
# Built-in math functions
myValues = (5, 10, 25)
# minimum, maximum
x = min(myValues)
y = max(myValues)
# 5, 25
print(x)
print(y)
# Absolute value
x = abs(-7.25)
# 7.25
print(x)
# x to the power of y
z = pow(4,3)
# 64
print(z)
print("----")
# Imported math functions
import math
x = math.sqrt(64)
print(x)
# Ceiling = round up to nearest integer
# Floor = round down to nearest integer
x = math.ceil(1.4)
y = math.floor(1.4)
# 2, 1
print(x)
print(y)
# PI is a constant (3.14....)
x = math.pi
# 3.14....
print(x)
myValues = (5, 10, 25)
# minimum, maximum
x = min(myValues)
y = max(myValues)
# 5, 25
print(x)
print(y)
# Absolute value
x = abs(-7.25)
# 7.25
print(x)
# x to the power of y
z = pow(4,3)
# 64
print(z)
print("----")
# Imported math functions
import math
x = math.sqrt(64)
print(x)
# Ceiling = round up to nearest integer
# Floor = round down to nearest integer
x = math.ceil(1.4)
y = math.floor(1.4)
# 2, 1
print(x)
print(y)
# PI is a constant (3.14....)
x = math.pi
# 3.14....
print(x)
Python - Dates
import datetime
x = datetime.datetime.now()
# 2022-07-02 18:23:57.824673
print(x)
# The datetime module has many methods, here are some
# 2022
print(x.year)
# Saturday
print(x.strftime("%A"))
# Create a date object
x = datetime.datetime(2020, 5, 17)
# 2020-05-17 00:00:00
print(x)
# Format date objects into readable strings via strftime()
# Good listing at w3schools:
# https://www.w3schools.com/python/python_datetime.asp
x = datetime.datetime(2018, 6, 1)
# %B = Month name
# June
print(x.strftime("%B"))
# %Y = year
# 2018
print(x.strftime("%Y"))
# %m = month as a two digit number
# 06
print(x.strftime("%m"))
# %d = day as a two digit number
# 01
print(x.strftime("%d"))
# There's probably a better way to do it than this
myFormattedDate = x.strftime("%Y") + "-" \
+ x.strftime("%m") + "-" \
+ x.strftime("%d")
# 2018-06-01
print(myFormattedDate)
# This works the same way
myFormattedDate = x.strftime('%Y-%m-%d')
print(myFormattedDate)
# Today's date
# 2022-07-02 18:42:02.671457
print(datetime.datetime.today())
# 2022-07-02
print(datetime.datetime.today().strftime('%Y-%m-%d'))
x = datetime.datetime.now()
# 2022-07-02 18:23:57.824673
print(x)
# The datetime module has many methods, here are some
# 2022
print(x.year)
# Saturday
print(x.strftime("%A"))
# Create a date object
x = datetime.datetime(2020, 5, 17)
# 2020-05-17 00:00:00
print(x)
# Format date objects into readable strings via strftime()
# Good listing at w3schools:
# https://www.w3schools.com/python/python_datetime.asp
x = datetime.datetime(2018, 6, 1)
# %B = Month name
# June
print(x.strftime("%B"))
# %Y = year
# 2018
print(x.strftime("%Y"))
# %m = month as a two digit number
# 06
print(x.strftime("%m"))
# %d = day as a two digit number
# 01
print(x.strftime("%d"))
# There's probably a better way to do it than this
myFormattedDate = x.strftime("%Y") + "-" \
+ x.strftime("%m") + "-" \
+ x.strftime("%d")
# 2018-06-01
print(myFormattedDate)
# This works the same way
myFormattedDate = x.strftime('%Y-%m-%d')
print(myFormattedDate)
# Today's date
# 2022-07-02 18:42:02.671457
print(datetime.datetime.today())
# 2022-07-02
print(datetime.datetime.today().strftime('%Y-%m-%d'))
Python - Modules
Modules are like libraries.
They let you tap into other people's code and packages.
For example, say there is a file named "mymodule.py" with these contents (between the dashed lines):
---------
def greeting(name):
print("Hello, " + name)
---------
We would then access the module with the code below:
import mymodule
# Hello, John
mymodule.greeting("John")
Here is how to access variables stored in a module:
Filename: mymodule.py:
---------
person1 = {
"name": "John",
"age": 36,
"country": "Norway"
}
---------
How I would use it:
import mymodule
a = mymodule.person1["age"]
#36
print(a)
There are built-in modules. For example, platform
We can display a list of functions in a module like this:
import platform
x = dir(platform)
print(x)
Python - Scope
# Variables are scoped
# It is only available from inside region it is created
def myfunc():
print("Inside function")
x = 300
print(x)
print("Exiting function")
# 300
myfunc()
# Next line fails
#print(x)
print("----")
# This sequence sets and displays a variable
# It then calls a function which *looks* like it messes with same variable
# But it shows that when we exit the function,
# the variable is intact
x = 7
# 7
print(x)
# From inside function, we will see 300
myfunc()
# 7
print(x)
print("=======")
# This shows that outer variables can be accessed from inner regions
# Define a global variable
outer = "my outer variable"
def myfunc():
inner = "my inner variable"
# Works as expected - we can see both
print(outer)
print(inner)
myfunc()
print("<><><><><><><>")
# Use global keyword to declare a variable global from within an inner region
def myfunc():
global surprise
surprise = 777
myfunc()
# the surprise variable is now treated as a global
# 777
print(surprise)
print("<><><><><><><>")
# We can change global variables from within an inner region
# by using the global keyword
x = 300
def myfunc():
global x
x = 200
myfunc()
# 200
print(x)
# It is only available from inside region it is created
def myfunc():
print("Inside function")
x = 300
print(x)
print("Exiting function")
# 300
myfunc()
# Next line fails
#print(x)
print("----")
# This sequence sets and displays a variable
# It then calls a function which *looks* like it messes with same variable
# But it shows that when we exit the function,
# the variable is intact
x = 7
# 7
print(x)
# From inside function, we will see 300
myfunc()
# 7
print(x)
print("=======")
# This shows that outer variables can be accessed from inner regions
# Define a global variable
outer = "my outer variable"
def myfunc():
inner = "my inner variable"
# Works as expected - we can see both
print(outer)
print(inner)
myfunc()
print("<><><><><><><>")
# Use global keyword to declare a variable global from within an inner region
def myfunc():
global surprise
surprise = 777
myfunc()
# the surprise variable is now treated as a global
# 777
print(surprise)
print("<><><><><><><>")
# We can change global variables from within an inner region
# by using the global keyword
x = 300
def myfunc():
global x
x = 200
myfunc()
# 200
print(x)
Python - Classes and Inheritance
class MyClass:
x = 5
p1 = MyClass()
# 5
print(p1.x)
print("---")
# built-in __init__() function is called on object instatiation
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
p1 = Person("John", 36)
# John, 36
print(p1.name)
print(p1.age)
print("===")
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def myfunc(self):
print("Hello my name is " + self.name)
p1 = Person("John", 36)
# Hello my name is John
p1.myfunc()
#36
print(p1.age)
# Set property
p1.age = 40
# 40
print(p1.age)
# <class '__main__.Person'>
# So this tells us it is an object of type Person
print (type(p1))
# Delete an object
del p1
# A class with no definition
class Person:
pass
print("******")
# Inheritance, Parents, Children
# First define a class
class Person:
def __init__(self, fname, lname):
self.firstname = fname
self.lastname = lname
def printname(self):
print(self.firstname, self.lastname)
x = Person("John", "Doe")
# John Doe
x.printname()
# Now create a child of the parent
class Student(Person):
pass
y = Student("Sarah", "Smith")
# Sarah Smith
y.printname()
print("------------")
# Notice that we never defined a method called printname() in the child class
# But the Student inherited the methods of the parent class (Person)
class Student(Person):
def __init__(self, fname, lname):
#add properties here
pass
y = Student("Sarah", "Smith")
# This next line breaks because we over-rode the inheritance of the parent
# by defining our own __init__() function in the chil
# In other words, the child's __init__() function overrides the inheritance
# of the parent's __init()__ function
#y.printname()
# We fix it by adding a call to the parent's __init__() function
class Student(Person):
def __init__(self, fname, lname):
Person.__init__(self, fname, lname)
y = Student("Sarah", "Smith")
# Now this works
# Sarah Smith
y.printname()
print("------------")
# By using super() we automatically inherit the methods and properties from parent
class Student(Person):
def __init__(self, fname, lname):
super().__init__(fname, lname)
self.graduationyear = 2019
y = Student("Sarah", "Smith")
# Sarah Smith
y.printname()
# 2019
print (y.graduationyear)
print("============")
# Make graduationyear a variable
class Student(Person):
def __init__(self, fname, lname, year):
super().__init__(fname, lname)
self.graduationyear = year
y = Student("Sarah", "Smith", 2022)
# Sarah Smith
y.printname()
# 2022
print (y.graduationyear)
print("<><><><><><>")
# Add a method
class Student(Person):
def __init__(self, fname, lname, year):
super().__init__(fname, lname)
self.graduationyear = year
def welcome(self):
print("Welcome", self.firstname, self.lastname, "to the class of", self.graduationyear)
x = 5
p1 = MyClass()
# 5
print(p1.x)
print("---")
# built-in __init__() function is called on object instatiation
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
p1 = Person("John", 36)
# John, 36
print(p1.name)
print(p1.age)
print("===")
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def myfunc(self):
print("Hello my name is " + self.name)
p1 = Person("John", 36)
# Hello my name is John
p1.myfunc()
#36
print(p1.age)
# Set property
p1.age = 40
# 40
print(p1.age)
# <class '__main__.Person'>
# So this tells us it is an object of type Person
print (type(p1))
# Delete an object
del p1
# A class with no definition
class Person:
pass
print("******")
# Inheritance, Parents, Children
# First define a class
class Person:
def __init__(self, fname, lname):
self.firstname = fname
self.lastname = lname
def printname(self):
print(self.firstname, self.lastname)
x = Person("John", "Doe")
# John Doe
x.printname()
# Now create a child of the parent
class Student(Person):
pass
y = Student("Sarah", "Smith")
# Sarah Smith
y.printname()
print("------------")
# Notice that we never defined a method called printname() in the child class
# But the Student inherited the methods of the parent class (Person)
class Student(Person):
def __init__(self, fname, lname):
#add properties here
pass
y = Student("Sarah", "Smith")
# This next line breaks because we over-rode the inheritance of the parent
# by defining our own __init__() function in the chil
# In other words, the child's __init__() function overrides the inheritance
# of the parent's __init()__ function
#y.printname()
# We fix it by adding a call to the parent's __init__() function
class Student(Person):
def __init__(self, fname, lname):
Person.__init__(self, fname, lname)
y = Student("Sarah", "Smith")
# Now this works
# Sarah Smith
y.printname()
print("------------")
# By using super() we automatically inherit the methods and properties from parent
class Student(Person):
def __init__(self, fname, lname):
super().__init__(fname, lname)
self.graduationyear = 2019
y = Student("Sarah", "Smith")
# Sarah Smith
y.printname()
# 2019
print (y.graduationyear)
print("============")
# Make graduationyear a variable
class Student(Person):
def __init__(self, fname, lname, year):
super().__init__(fname, lname)
self.graduationyear = year
y = Student("Sarah", "Smith", 2022)
# Sarah Smith
y.printname()
# 2022
print (y.graduationyear)
print("<><><><><><>")
# Add a method
class Student(Person):
def __init__(self, fname, lname, year):
super().__init__(fname, lname)
self.graduationyear = year
def welcome(self):
print("Welcome", self.firstname, self.lastname, "to the class of", self.graduationyear)
Python - Functions
# Functions
def my_function():
print("Hello from a function")
my_function()
# So apparently we can redefine functions
# Notice the function name below is the same one defined above
def my_function(fname):
print(fname + " Smith")
my_function("John")
my_function("Jane")
# Two args
def my_function(fname, lname):
print (fname + " " + lname)
my_function("John", "Doe")
my_function("Jane", "Smith")
# This will break because not right number of args
#my_function("Pat")
#my_function("Richard", "Paul", "Astley")
print ("---")
def my_function(*kids):
for kid in kids:
print(kid)
# Prints John, Jane, Richard
my_function("John", "Jane", "Richard")
print ("---")
# You can specify arguments instead of relying on order
def my_function(child3, child2, child1):
print("The youngest child is " + child3)
# Displays Linus
my_function(child1 = "Emil", child2 = "Tobias", child3 = "Linus")
print ("===")
# Default parameter values
def my_function(country = "Norway"):
print("I am from " + country)
my_function("Sweden")
my_function("India")
# Displays "I am from Norway"
my_function()
my_function("Brazil")
# Return a value
def my_function(x):
return 5 * x
# Displays 15, 25, 45
print(my_function(3))
print(my_function(5))
print(my_function(9))
print ("---")
# Empty functions
def myfunction():
pass
def my_function():
print("Hello from a function")
my_function()
# So apparently we can redefine functions
# Notice the function name below is the same one defined above
def my_function(fname):
print(fname + " Smith")
my_function("John")
my_function("Jane")
# Two args
def my_function(fname, lname):
print (fname + " " + lname)
my_function("John", "Doe")
my_function("Jane", "Smith")
# This will break because not right number of args
#my_function("Pat")
#my_function("Richard", "Paul", "Astley")
print ("---")
def my_function(*kids):
for kid in kids:
print(kid)
# Prints John, Jane, Richard
my_function("John", "Jane", "Richard")
print ("---")
# You can specify arguments instead of relying on order
def my_function(child3, child2, child1):
print("The youngest child is " + child3)
# Displays Linus
my_function(child1 = "Emil", child2 = "Tobias", child3 = "Linus")
print ("===")
# Default parameter values
def my_function(country = "Norway"):
print("I am from " + country)
my_function("Sweden")
my_function("India")
# Displays "I am from Norway"
my_function()
my_function("Brazil")
# Return a value
def my_function(x):
return 5 * x
# Displays 15, 25, 45
print(my_function(3))
print(my_function(5))
print(my_function(9))
print ("---")
# Empty functions
def myfunction():
pass
Python - For loops
# Prints each item, one on each line
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
# Prints each letter in 'banana', one on each line
for x in "banana":
print(x)
print ("---")
# Break exits the loop
# Displays apple, then banana, and then breaks out of the loop
# (So it never gets to cherry)
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
if x == "banana":
break
print ("---")
# This does the same thing but the break is before the print statement
# So we only see "apple"
# Recall that we are breaking out of the FOR loop, not just the IF condition
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x == "banana":
break
print(x)
print ("===")
# continue says stop the current iteration of the loop
# and continue with the next one
# apple
# rotten!
# cherry
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x == "banana":
print("rotten!")
continue
print(x)
# Displays 0 through 5
for x in range(6):
print(x)
print ("===")
# Displays 2 through 5
for x in range(2, 6):
print(x)
print ("===")
# Weird
# Start at 2
# Keep going up to right before 30
# Increment by the third argument: 3
# So it displays 2, 5, 8, 11, 14, 17, 20, 23, 26, and 29
for x in range(2, 30, 3):
print(x)
print("<><><><>")
# Displays 0 through 5 and then executes the else statement
# The else keyword says run this block of code when the loop statement is finished
for x in range(6):
print(x)
else:
print("Finally finished!")
print("<><><><>")
# Displays 0
for x in range(1):
print(x)
print("<><><><>")
# Displays 0, 1
for x in range(2):
print(x)
print("<><><><>")
# Displays 0, done
for x in range(1):
print(x)
else:
print("done")
print("<*><*><*><*>")
# Displays done
for x in range(0):
print(x)
else:
print("done")
# The break statement completely exits out of the for loop
# It will not execute the else block
# Displays 0, 1, 2
print ("----")
for x in range(6):
if x == 3: break
print(x)
else:
print("Finally finished!")
# Fun with nested loops
# Displays red apple, red banana, red cherry, big x3, tasty x3
adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]
for x in adj:
for y in fruits:
print(x, y)
# What you have to do if no content in a for loop
for x in [0, 1, 2]:
pass
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
# Prints each letter in 'banana', one on each line
for x in "banana":
print(x)
print ("---")
# Break exits the loop
# Displays apple, then banana, and then breaks out of the loop
# (So it never gets to cherry)
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
if x == "banana":
break
print ("---")
# This does the same thing but the break is before the print statement
# So we only see "apple"
# Recall that we are breaking out of the FOR loop, not just the IF condition
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x == "banana":
break
print(x)
print ("===")
# continue says stop the current iteration of the loop
# and continue with the next one
# apple
# rotten!
# cherry
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x == "banana":
print("rotten!")
continue
print(x)
# Displays 0 through 5
for x in range(6):
print(x)
print ("===")
# Displays 2 through 5
for x in range(2, 6):
print(x)
print ("===")
# Weird
# Start at 2
# Keep going up to right before 30
# Increment by the third argument: 3
# So it displays 2, 5, 8, 11, 14, 17, 20, 23, 26, and 29
for x in range(2, 30, 3):
print(x)
print("<><><><>")
# Displays 0 through 5 and then executes the else statement
# The else keyword says run this block of code when the loop statement is finished
for x in range(6):
print(x)
else:
print("Finally finished!")
print("<><><><>")
# Displays 0
for x in range(1):
print(x)
print("<><><><>")
# Displays 0, 1
for x in range(2):
print(x)
print("<><><><>")
# Displays 0, done
for x in range(1):
print(x)
else:
print("done")
print("<*><*><*><*>")
# Displays done
for x in range(0):
print(x)
else:
print("done")
# The break statement completely exits out of the for loop
# It will not execute the else block
# Displays 0, 1, 2
print ("----")
for x in range(6):
if x == 3: break
print(x)
else:
print("Finally finished!")
# Fun with nested loops
# Displays red apple, red banana, red cherry, big x3, tasty x3
adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]
for x in adj:
for y in fruits:
print(x, y)
# What you have to do if no content in a for loop
for x in [0, 1, 2]:
pass
Python - While loops
# While loop
# Displays 1 through 5
i = 1
while i < 6:
print(i)
i += 1
print("---")
# break statement
# Displays 1 through 3
i = 1
while i < 6:
print(i)
if i == 3:
break
i += 1
print("---")
# continue statement
# Displays 1 through 6
i = 0
while i < 6:
i += 1
if i == 3:
continue
print(i)
print("---------")
# Displays 1 through 5, and then executes the else statement
# While ... Else
i = 1
while i < 6:
print(i)
i += 1
else:
print("i is no longer less than 6")
# Displays 1 through 5
i = 1
while i < 6:
print(i)
i += 1
print("---")
# break statement
# Displays 1 through 3
i = 1
while i < 6:
print(i)
if i == 3:
break
i += 1
print("---")
# continue statement
# Displays 1 through 6
i = 0
while i < 6:
i += 1
if i == 3:
continue
print(i)
print("---------")
# Displays 1 through 5, and then executes the else statement
# While ... Else
i = 1
while i < 6:
print(i)
i += 1
else:
print("i is no longer less than 6")
Python - If... Else
# Fun with if conditions
a = 33
b = 200
if b > a:
print("b is greater than a")
a = 33
b = 33
if b > a:
print("b is greater than a")
elif a == b:
print("a and b are equal")
a = 200
b = 33
if b > a:
print("b is greater than a")
elif a == b:
print("a and b are equal")
else:
print("a is greater than b")
a = 200
b = 33
if b > a:
print("b is greater than a")
else:
print("b is not greater than a")
print("---")
a = 200
b = 33
# Short Hand If
if a > b: print("a is greater than b")
# Short Hand If ... Else
a = 2
b = 330
print("A") if a > b else print("B")
print("-----")
# AND
a = 200
b = 33
c = 500
if a > b and c > a:
print("Both conditions are True")
# Parentheses seems to work
if (a > b) and (c > a):
print("Both conditions are True")
a = 200
b = 33
c = 500
if a > b or a > c:
print("At least one of the conditions is True")
print("<><><><>")
x = 41
if x > 10:
print("Above ten,")
if x > 20:
print("and also above 20!")
else:
print("but not above 20.")
# If statements cannot be empty but we can use pass
a = 33
b = 200
if b > a:
# Do nothing
# Python won't let me just have comments here
# I have to use the pass statement
pass
a = 33
b = 200
if b > a:
print("b is greater than a")
a = 33
b = 33
if b > a:
print("b is greater than a")
elif a == b:
print("a and b are equal")
a = 200
b = 33
if b > a:
print("b is greater than a")
elif a == b:
print("a and b are equal")
else:
print("a is greater than b")
a = 200
b = 33
if b > a:
print("b is greater than a")
else:
print("b is not greater than a")
print("---")
a = 200
b = 33
# Short Hand If
if a > b: print("a is greater than b")
# Short Hand If ... Else
a = 2
b = 330
print("A") if a > b else print("B")
print("-----")
# AND
a = 200
b = 33
c = 500
if a > b and c > a:
print("Both conditions are True")
# Parentheses seems to work
if (a > b) and (c > a):
print("Both conditions are True")
a = 200
b = 33
c = 500
if a > b or a > c:
print("At least one of the conditions is True")
print("<><><><>")
x = 41
if x > 10:
print("Above ten,")
if x > 20:
print("and also above 20!")
else:
print("but not above 20.")
# If statements cannot be empty but we can use pass
a = 33
b = 200
if b > a:
# Do nothing
# Python won't let me just have comments here
# I have to use the pass statement
pass
Python - Dictionaries
# Dictionaries
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
# {'brand': 'Ford', 'model': 'Mustang', 'year': 1964}
print(thisdict)
# Ford
print(thisdict["brand"])
# 3
print(len(thisdict))
mymodel = thisdict["model"]
# Mustang
print (mymodel)
mykeys = thisdict.keys()
# dict_keys(['brand', 'model', 'year'])
print (mykeys)
myvalues = thisdict.values()
# dict_values(['Ford', 'Mustang', 1964])
print (myvalues)
myitems = thisdict.items()
# dict_items([('brand', 'Ford'), ('model', 'Mustang'), ('year', 1964)])
print (myitems)
print("---")
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
# Change a value
thisdict["year"] = 2018
# 2018
print(thisdict["year"])
thisdict.update({"year": 2020})
# 2020
print(thisdict["year"])
# Add an item
thisdict["color"] = "red"
# {'brand': 'Ford', 'model': 'Mustang', 'year': 2020, 'color': 'red'}
print (thisdict)
# This updates the value
# If the value does not exist, it will add it
thisdict.update({"color": "blue"})
thisdict.update({"wheels": 4})
# {'brand': 'Ford', 'model': 'Mustang', 'year': 2020, 'color': 'blue', 'wheels': 4}
print (thisdict)
# Pop removes the last inserted item and returns the item
# 5
print(len(thisdict))
x = thisdict.popitem()
# ('wheels', 4)
print(x)
# 4
print(len(thisdict))
del thisdict["color"]
del thisdict["model"]
# {'brand': 'Ford', 'year': 2020}
print(thisdict)
# Clear the dictionary of all items
thisdict.clear()
# {}
print(thisdict)
# Remove the dictionary completely
del thisdict
# Next line will cause an error
#print(thisdict)
print("---")
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
# This will loop through and show the keys
# brand
# model
# year
for x in thisdict:
print(x)
# This will loop through and show the values
# Ford
# Mustang
# 1964
for x in thisdict:
print(thisdict[x])
print("---")
# Ford
# Mustang
# 1964
for x in thisdict.values():
print(x)
# brand
# model
# year
for x in thisdict.keys():
print(x)
print("---")
# Gnarly - loop through both keys and values
# brand Ford
# model Mustang
# year 1964
for x, y in thisdict.items():
print(x, y)
print("======")
# Copy a dictionary
# Truly a copy and not a reference
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
mydict = thisdict.copy()
print(mydict)
print("---")
mydict["year"] = 2020
print(mydict)
print(thisdict)
print("---")
myfamily = {
"child1" : {
"name" : "Emil",
"year" : 2004
},
"child2" : {
"name" : "Tobias",
"year" : 2007
},
"child3" : {
"name" : "Linus",
"year" : 2011
}
}
# <class 'dict'>
print(type(myfamily))
# <class 'dict'>
print(type(myfamily["child1"]))
# {'name': 'Emil', 'year': 2004}
print(myfamily["child1"])
# Emil
print(myfamily["child1"]["name"])
# <class 'str'>
print(type(myfamily["child1"]["name"]))
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
# {'brand': 'Ford', 'model': 'Mustang', 'year': 1964}
print(thisdict)
# Ford
print(thisdict["brand"])
# 3
print(len(thisdict))
mymodel = thisdict["model"]
# Mustang
print (mymodel)
mykeys = thisdict.keys()
# dict_keys(['brand', 'model', 'year'])
print (mykeys)
myvalues = thisdict.values()
# dict_values(['Ford', 'Mustang', 1964])
print (myvalues)
myitems = thisdict.items()
# dict_items([('brand', 'Ford'), ('model', 'Mustang'), ('year', 1964)])
print (myitems)
print("---")
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
# Change a value
thisdict["year"] = 2018
# 2018
print(thisdict["year"])
thisdict.update({"year": 2020})
# 2020
print(thisdict["year"])
# Add an item
thisdict["color"] = "red"
# {'brand': 'Ford', 'model': 'Mustang', 'year': 2020, 'color': 'red'}
print (thisdict)
# This updates the value
# If the value does not exist, it will add it
thisdict.update({"color": "blue"})
thisdict.update({"wheels": 4})
# {'brand': 'Ford', 'model': 'Mustang', 'year': 2020, 'color': 'blue', 'wheels': 4}
print (thisdict)
# Pop removes the last inserted item and returns the item
# 5
print(len(thisdict))
x = thisdict.popitem()
# ('wheels', 4)
print(x)
# 4
print(len(thisdict))
del thisdict["color"]
del thisdict["model"]
# {'brand': 'Ford', 'year': 2020}
print(thisdict)
# Clear the dictionary of all items
thisdict.clear()
# {}
print(thisdict)
# Remove the dictionary completely
del thisdict
# Next line will cause an error
#print(thisdict)
print("---")
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
# This will loop through and show the keys
# brand
# model
# year
for x in thisdict:
print(x)
# This will loop through and show the values
# Ford
# Mustang
# 1964
for x in thisdict:
print(thisdict[x])
print("---")
# Ford
# Mustang
# 1964
for x in thisdict.values():
print(x)
# brand
# model
# year
for x in thisdict.keys():
print(x)
print("---")
# Gnarly - loop through both keys and values
# brand Ford
# model Mustang
# year 1964
for x, y in thisdict.items():
print(x, y)
print("======")
# Copy a dictionary
# Truly a copy and not a reference
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
mydict = thisdict.copy()
print(mydict)
print("---")
mydict["year"] = 2020
print(mydict)
print(thisdict)
print("---")
myfamily = {
"child1" : {
"name" : "Emil",
"year" : 2004
},
"child2" : {
"name" : "Tobias",
"year" : 2007
},
"child3" : {
"name" : "Linus",
"year" : 2011
}
}
# <class 'dict'>
print(type(myfamily))
# <class 'dict'>
print(type(myfamily["child1"]))
# {'name': 'Emil', 'year': 2004}
print(myfamily["child1"])
# Emil
print(myfamily["child1"]["name"])
# <class 'str'>
print(type(myfamily["child1"]["name"]))
Python - Set joins
# We can combine two sets with the union method
set1 = {"a", "b", "c"}
set2 = {1, 2, 3}
set3 = set1.union(set2)
# {'a', 1, 2, 3, 'c', 'b'}
print(set3)
# Add items from set2 to set1
set1.update(set2)
print (set1)
# Both union and update exclude duplicates
# intersection_update keeps only items that are present in both sets
x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}
x.intersection_update(y)
# {'apple'}
print(x)
# intersection() returns a NEW set of items present in both sets
x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}
z = x.intersection(y)
# {'apple'}
print(z)
# Keep all but NOT the duplicates
x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}
x.symmetric_difference_update(y)
# {'cherry', 'microsoft', 'google', 'banana'}
# Notice no 'apple'
print(x)
set1 = {"a", "b", "c"}
set2 = {1, 2, 3}
set3 = set1.union(set2)
# {'a', 1, 2, 3, 'c', 'b'}
print(set3)
# Add items from set2 to set1
set1.update(set2)
print (set1)
# Both union and update exclude duplicates
# intersection_update keeps only items that are present in both sets
x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}
x.intersection_update(y)
# {'apple'}
print(x)
# intersection() returns a NEW set of items present in both sets
x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}
z = x.intersection(y)
# {'apple'}
print(z)
# Keep all but NOT the duplicates
x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}
x.symmetric_difference_update(y)
# {'cherry', 'microsoft', 'google', 'banana'}
# Notice no 'apple'
print(x)
Python - Add and remove items from a set
# Sets are unordered, unchangeable, and unindexed
# Unchangeable means you cannot change the item in a set,
# but you CAN add or remove items from the set
# sets use curly brackets {}
myset = {"apple", "banana", "cherry"}
# No duplicates
myset = {"apple", "banana", "cherry", "apple"}
# {'apple', 'cherry', 'banana'}
print (myset)
# 3
print(len(myset))
# Loop through a set
for x in myset:
print(x)
# Add item
myset.add("orange")
# {'orange', 'banana', 'cherry', 'apple'}
print(myset)
# Add items from another set to myset
tropical = {"pineapple", "mango", "papaya"}
myset.update(tropical)
# {'orange', 'apple', 'mango', 'papaya', 'banana', 'pineapple', 'cherry'}
print(myset)
myset.remove("orange")
# {'pineapple', 'cherry', 'mango', 'papaya', 'banana', 'apple'}
# (notice no 'orange')
print(myset)
# If you try to remove the item and it does not exist,
# Python will raise an error
# myset.remove("orange")
# However, the discard() method will NOT raise an error
myset.discard("pineapple")
print(myset)
myset.discard("pineapple")
print(myset)
# Pop removes the last item from the set
# Its return value is the item removed
x = myset.pop()
print(myset)
print("Notice that ", x , " no longer exists in the set: " , myset)
print("---")
# Clear removes all items from the set
myset.clear()
print(myset)
del myset
# This would raise an error because the set no longer exists
#print(myset)
# Unchangeable means you cannot change the item in a set,
# but you CAN add or remove items from the set
# sets use curly brackets {}
myset = {"apple", "banana", "cherry"}
# No duplicates
myset = {"apple", "banana", "cherry", "apple"}
# {'apple', 'cherry', 'banana'}
print (myset)
# 3
print(len(myset))
# Loop through a set
for x in myset:
print(x)
# Add item
myset.add("orange")
# {'orange', 'banana', 'cherry', 'apple'}
print(myset)
# Add items from another set to myset
tropical = {"pineapple", "mango", "papaya"}
myset.update(tropical)
# {'orange', 'apple', 'mango', 'papaya', 'banana', 'pineapple', 'cherry'}
print(myset)
myset.remove("orange")
# {'pineapple', 'cherry', 'mango', 'papaya', 'banana', 'apple'}
# (notice no 'orange')
print(myset)
# If you try to remove the item and it does not exist,
# Python will raise an error
# myset.remove("orange")
# However, the discard() method will NOT raise an error
myset.discard("pineapple")
print(myset)
myset.discard("pineapple")
print(myset)
# Pop removes the last item from the set
# Its return value is the item removed
x = myset.pop()
print(myset)
print("Notice that ", x , " no longer exists in the set: " , myset)
print("---")
# Clear removes all items from the set
myset.clear()
print(myset)
del myset
# This would raise an error because the set no longer exists
#print(myset)
Friday, July 1, 2022
Python - Unpacking Tuples
# Python can "unpack" tuple items into variables like this:
fruits = ("apple", "banana", "cherry")
(green, yellow, red) = fruits
# apple
print(green)
# banana
print(yellow)
# cherry
print(red)
print("---")
# This will cause an error because there are
# too many values to unpack
# into only three variables
fruits = ("apple", "banana", "cherry", "strawberry", "raspberry")
#(green, yellow, red) = fruits
# So use an asterisk * instead
(green, yellow, *red) = fruits
# apple
print(green)
# banana
print(yellow)
# ['cherry', 'strawberry', 'raspberry']
print(red)
# All the remaining variables got shoved into a list called "red"
# <class 'list'>
print(type(red))
(green, *tropic, red) = fruits
print("---")
# apple
print(green)
# You will notice that Python assigned all the values to "tropic"
# except the last one so it matched the number of variables we provided.
# ['banana', 'cherry', 'strawberry']
print(tropic)
# raspberry
print(red)
fruits = ("apple", "banana", "cherry")
(green, yellow, red) = fruits
# apple
print(green)
# banana
print(yellow)
# cherry
print(red)
print("---")
# This will cause an error because there are
# too many values to unpack
# into only three variables
fruits = ("apple", "banana", "cherry", "strawberry", "raspberry")
#(green, yellow, red) = fruits
# So use an asterisk * instead
(green, yellow, *red) = fruits
# apple
print(green)
# banana
print(yellow)
# ['cherry', 'strawberry', 'raspberry']
print(red)
# All the remaining variables got shoved into a list called "red"
# <class 'list'>
print(type(red))
(green, *tropic, red) = fruits
print("---")
# apple
print(green)
# You will notice that Python assigned all the values to "tropic"
# except the last one so it matched the number of variables we provided.
# ['banana', 'cherry', 'strawberry']
print(tropic)
# raspberry
print(red)
Python - Tuples
# Lists use a square bracket []
# Tuples use parentheses ()
mylist = ["apple", "banana", "cherry", "apple"]
mytuple = ("apple", "banana", "cherry", "apple")
#<class 'list'>
#<class 'tuple'>
print (type(mylist))
print (type(mytuple))
# Length of tuple
# 4
print(len(mytuple))
# A tuple with only one item is a bit gnarly
# Notice the ending comma:
thistuple = ("apple",)
# If we forget the comma it is a string
myString = ("apple")
# <class 'str'>
print (type(myString))
# Since tuples are "unchangeable", we must jump thru some hoops
# if we *do* want to change one.
# Set up a tuple
x = ("apple", "banana", "cherry")
# Create a list type of the tuple
y = list(x)
# Make your change
y[1] = "kiwi"
# Now redefine the original tuple as a tuple
# but give it the contents of the changed list
x = tuple(y)
print(x)
# Adding items to an unchangeable tuple is similar
thistuple = ("apple", "banana", "cherry")
y = list(thistuple)
y.append("orange")
thistuple = tuple(y)
print(thistuple)
print("---")
# Removing an item from an unchangeable tuple is similar
thistuple = ("apple", "banana", "cherry")
y = list(thistuple)
y.remove("apple")
thistuple = tuple(y)
print(thistuple)
# Tuples use parentheses ()
mylist = ["apple", "banana", "cherry", "apple"]
mytuple = ("apple", "banana", "cherry", "apple")
#<class 'list'>
#<class 'tuple'>
print (type(mylist))
print (type(mytuple))
# Length of tuple
# 4
print(len(mytuple))
# A tuple with only one item is a bit gnarly
# Notice the ending comma:
thistuple = ("apple",)
# If we forget the comma it is a string
myString = ("apple")
# <class 'str'>
print (type(myString))
# Since tuples are "unchangeable", we must jump thru some hoops
# if we *do* want to change one.
# Set up a tuple
x = ("apple", "banana", "cherry")
# Create a list type of the tuple
y = list(x)
# Make your change
y[1] = "kiwi"
# Now redefine the original tuple as a tuple
# but give it the contents of the changed list
x = tuple(y)
print(x)
# Adding items to an unchangeable tuple is similar
thistuple = ("apple", "banana", "cherry")
y = list(thistuple)
y.append("orange")
thistuple = tuple(y)
print(thistuple)
print("---")
# Removing an item from an unchangeable tuple is similar
thistuple = ("apple", "banana", "cherry")
y = list(thistuple)
y.remove("apple")
thistuple = tuple(y)
print(thistuple)
Python - Join Lists
# Multiple ways to join two lists
# One way
list1 = ["a", "b", "c"]
list2 = [1, 2, 3]
list3 = list1 + list2
print(list3)
# Another way
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
for x in list2:
list1.append(x)
print(list1)
# A third way
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
list1.extend(list2)
print(list1)
# One way
list1 = ["a", "b", "c"]
list2 = [1, 2, 3]
list3 = list1 + list2
print(list3)
# Another way
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
for x in list2:
list1.append(x)
print(list1)
# A third way
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
list1.extend(list2)
print(list1)
Python - Copy lists
list1 = ["apple", "banana", "cherry"]
list2 = list1
# Right now these are identical
print (list1)
print (list2)
list1.append('orange')
# Notice that the new item is also added to list2
# ['apple', 'banana', 'cherry', 'orange']
print (list1)
print (list2)
# I think this means list2 is a pointer referencing list1.
# So changes made to list1 are seen in list2,
# and vice-versa.
list2.pop()
# ['apple', 'banana', 'cherry']
print (list1)
print (list2)
# So we need a way to actually *copy* the list
list2 = list1.copy()
list1.append('orange')
list2.pop()
# Now we're cooking with gas
# ['apple', 'banana', 'cherry', 'orange']
# ['apple', 'banana']
print (list1)
print (list2)
print("---")
# Another way to copy a list
list1 = ["apple", "banana", "cherry"]
list3 = list(list1)
# ['apple', 'banana', 'cherry']
print (list3)
list3.insert(2, "popsicle")
#['apple', 'banana', 'cherry']
#['apple', 'banana', 'popsicle', 'cherry']
print (list1)
print (list3)
list2 = list1
# Right now these are identical
print (list1)
print (list2)
list1.append('orange')
# Notice that the new item is also added to list2
# ['apple', 'banana', 'cherry', 'orange']
print (list1)
print (list2)
# I think this means list2 is a pointer referencing list1.
# So changes made to list1 are seen in list2,
# and vice-versa.
list2.pop()
# ['apple', 'banana', 'cherry']
print (list1)
print (list2)
# So we need a way to actually *copy* the list
list2 = list1.copy()
list1.append('orange')
list2.pop()
# Now we're cooking with gas
# ['apple', 'banana', 'cherry', 'orange']
# ['apple', 'banana']
print (list1)
print (list2)
print("---")
# Another way to copy a list
list1 = ["apple", "banana", "cherry"]
list3 = list(list1)
# ['apple', 'banana', 'cherry']
print (list3)
list3.insert(2, "popsicle")
#['apple', 'banana', 'cherry']
#['apple', 'banana', 'popsicle', 'cherry']
print (list1)
print (list3)
Python - Sort lists
# Sort a list
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort()
# ['banana', 'kiwi', 'mango', 'orange', 'pineapple']
print(thislist)
# I guess Python knows to sort numerically instead of alphabetically
# if the items in the list are all numeric
thislist = [100, 50, 65, 82, 23]
thislist.sort()
# [23, 50, 65, 82, 100]
print(thislist)
# Sort descending
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort(reverse = True)
# ['pineapple', 'orange', 'mango', 'kiwi', 'banana']
print(thislist)
# Sort numbers descending
thislist = [100, 50, 65, 82, 23]
thislist.sort(reverse = True)
# [100, 82, 65, 50, 23]
print(thislist)
# Sort is case sensitive
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort()
# ['Kiwi', 'Orange', 'banana', 'cherry']
print(thislist)
# How to do a case insensitive sort
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort(key = str.lower)
# ['banana', 'cherry', 'Kiwi', 'Orange']
print(thislist)
# This reverses the items in a list
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.reverse()
# ['cherry', 'Kiwi', 'Orange', 'banana']
print(thislist)
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort()
# ['banana', 'kiwi', 'mango', 'orange', 'pineapple']
print(thislist)
# I guess Python knows to sort numerically instead of alphabetically
# if the items in the list are all numeric
thislist = [100, 50, 65, 82, 23]
thislist.sort()
# [23, 50, 65, 82, 100]
print(thislist)
# Sort descending
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort(reverse = True)
# ['pineapple', 'orange', 'mango', 'kiwi', 'banana']
print(thislist)
# Sort numbers descending
thislist = [100, 50, 65, 82, 23]
thislist.sort(reverse = True)
# [100, 82, 65, 50, 23]
print(thislist)
# Sort is case sensitive
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort()
# ['Kiwi', 'Orange', 'banana', 'cherry']
print(thislist)
# How to do a case insensitive sort
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort(key = str.lower)
# ['banana', 'cherry', 'Kiwi', 'Orange']
print(thislist)
# This reverses the items in a list
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.reverse()
# ['cherry', 'Kiwi', 'Orange', 'banana']
print(thislist)
Python - List Comprehension
# List comprehension is weird
# It is a shortcut
# W3Schools gives an example where you have a list of fruits
# and want to return a new list containing only the fruits
# with the letter "a" in the name
# The typical way:
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = []
# This will loop through all the items in the list "fruits"
for x in fruits:
# If the letter "a" is in the item
if "a" in x:
# Then add this item to the new list
newlist.append(x)
# ['apple', 'banana', 'mango']
print(newlist)
# We can use list comprehension to do the same thing
# but with fewer lines of code
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]
# ['apple', 'banana', 'mango']
print(newlist)
# You can also manipulate the items in the list before you return
# the result like this:
newlist = [x.upper() for x in fruits]
# ['APPLE', 'BANANA', 'CHERRY', 'KIWI', 'MANGO']
print(newlist)
# It is a shortcut
# W3Schools gives an example where you have a list of fruits
# and want to return a new list containing only the fruits
# with the letter "a" in the name
# The typical way:
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = []
# This will loop through all the items in the list "fruits"
for x in fruits:
# If the letter "a" is in the item
if "a" in x:
# Then add this item to the new list
newlist.append(x)
# ['apple', 'banana', 'mango']
print(newlist)
# We can use list comprehension to do the same thing
# but with fewer lines of code
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]
# ['apple', 'banana', 'mango']
print(newlist)
# You can also manipulate the items in the list before you return
# the result like this:
newlist = [x.upper() for x in fruits]
# ['APPLE', 'BANANA', 'CHERRY', 'KIWI', 'MANGO']
print(newlist)
Python - Loop through lists
# Loop through a list
thislist = ["apple", "banana", "cherry"]
# This will show each item in the list, one line per item
# apple
# banana
# cherry
for x in thislist:
print(x)
# 3
print (len(thislist))
# range(0, 3)
print (range(len(thislist)))
print("---")
# This will print each index number of the list: 0, 1, and 2
for i in range(len(thislist)):
print(i)
# This will print each item in the list
# by specifing each index number in the list:
for i in range(len(thislist)):
print(thislist[i])
print("---")
# Loop through with a while loop
thislist = ["apple", "banana", "cherry"]
i = 0
while i < len(thislist):
print(thislist[i])
i += 1
# Looping with "List Comprehension"
thislist = ["apple", "banana", "cherry"]
[print(x) for x in thislist]
thislist = ["apple", "banana", "cherry"]
# This will show each item in the list, one line per item
# apple
# banana
# cherry
for x in thislist:
print(x)
# 3
print (len(thislist))
# range(0, 3)
print (range(len(thislist)))
print("---")
# This will print each index number of the list: 0, 1, and 2
for i in range(len(thislist)):
print(i)
# This will print each item in the list
# by specifing each index number in the list:
for i in range(len(thislist)):
print(thislist[i])
print("---")
# Loop through with a while loop
thislist = ["apple", "banana", "cherry"]
i = 0
while i < len(thislist):
print(thislist[i])
i += 1
# Looping with "List Comprehension"
thislist = ["apple", "banana", "cherry"]
[print(x) for x in thislist]
Python - Add or remove items from a list
# Add an item to a list
myList = ["apple", "banana", "cherry"]
myList.append("orange")
print(myList)
# Add multiple items to a list
myList = ["apple", "banana", "cherry"]
tropical = ["mango", "pineapple", "papaya"]
myList.extend(tropical)
# ['apple', 'banana', 'cherry', 'mango', 'pineapple', 'papaya']
print(myList)
# Insert an item mid-list
myList = ["apple", "banana", "cherry"]
myList.insert(1,"orange")
# ['apple', 'orange', 'banana', 'cherry']
print(myList)
# Remove an item from a list
thislist = ["apple", "banana", "cherry"]
thislist.remove("banana")
# ['apple', 'cherry']
print(thislist)
# Pop method to remove the list item from a list
thislist = ["apple", "banana", "cherry"]
thislist.pop()
# ['apple', 'banana']
print(thislist)
# Pop method with an index value targets that item in list
thislist = ["apple", "banana", "cherry"]
thislist.pop(1)
# ['apple', 'cherry']
print(thislist)
# Delete method
thislist = ["apple", "banana", "cherry"]
del thislist[0]
# ['banana', 'cherry']
print(thislist)
# Delete the entire list
thislist = ["apple", "banana", "cherry"]
del thislist
# This will generate an error because the list itself is deleted,
# not simply emptied of all items.
#print(thislist)
# This is the way to empty all items from a list
thislist = ["apple", "banana", "cherry"]
thislist.clear()
# []
print(thislist)
myList = ["apple", "banana", "cherry"]
myList.append("orange")
print(myList)
# Add multiple items to a list
myList = ["apple", "banana", "cherry"]
tropical = ["mango", "pineapple", "papaya"]
myList.extend(tropical)
# ['apple', 'banana', 'cherry', 'mango', 'pineapple', 'papaya']
print(myList)
# Insert an item mid-list
myList = ["apple", "banana", "cherry"]
myList.insert(1,"orange")
# ['apple', 'orange', 'banana', 'cherry']
print(myList)
# Remove an item from a list
thislist = ["apple", "banana", "cherry"]
thislist.remove("banana")
# ['apple', 'cherry']
print(thislist)
# Pop method to remove the list item from a list
thislist = ["apple", "banana", "cherry"]
thislist.pop()
# ['apple', 'banana']
print(thislist)
# Pop method with an index value targets that item in list
thislist = ["apple", "banana", "cherry"]
thislist.pop(1)
# ['apple', 'cherry']
print(thislist)
# Delete method
thislist = ["apple", "banana", "cherry"]
del thislist[0]
# ['banana', 'cherry']
print(thislist)
# Delete the entire list
thislist = ["apple", "banana", "cherry"]
del thislist
# This will generate an error because the list itself is deleted,
# not simply emptied of all items.
#print(thislist)
# This is the way to empty all items from a list
thislist = ["apple", "banana", "cherry"]
thislist.clear()
# []
print(thislist)
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