This post is a record made while learning Chapter 5 “If Statements” in Eric Matthes’s book, Python Crash Course.
Conditional tests (i.e. Boolean expression)
Check equality for strings: ==
Check equality for strings by equality operator (double equal sign, ==):
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| cars = ['audi', 'bmw', 'subaru', 'toyota']
for car in cars:
if car == 'bmw': # This is a "conditional test"
print(car.upper())
else:
print(car.title())
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| Audi
BMW
Subaru
Toyota
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Testing for equality is case sensitive in Python. We can use lower() method to ignore how the value is formatted, namely case insensitive, for example:
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| car = 'Audi'
print(car == 'audi')
print(car.lower() == 'audi')
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And, lower() method won’t change the original value of variable car.
Check inequality for strings: !=
Check inequality for strings by inequality operator (an exclamation point and an equal sign, !=):
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| requested_topping = 'mushrooms'
if requested_topping != 'anchovies':
print("Hold the anchovies!")
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Numerical comparisons: ==, <, <=, >, and >=
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| age = 19
age == 21, age < 21, age <= 21, age > 21, age >= 21
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| (False, True, True, False, False)
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Check multiple conditions: and, or
Keyword and:
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| age_0, age_1 = 22, 18
print((age_0 >= 21) and (age_1 >= 21))
age_0, age_1 = 22, 22
print((age_0 >= 21) and (age_1 >= 21))
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Parentheses for individual tests are not required, just to improve readability.
Keyword or:
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| age_0, age_1 = 22, 18
print(age_0 >= 21 or age_1 >= 21)
age_0, age_1 = 18, 18
print(age_0 >= 21 or age_1 >= 21)
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Check whether a value is or isn’t in a list: in, not in
To find out whether a particular value is (or isn’t) already in a list, use the keyword in (or not in):
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| requested_toppings = ['mushrooms', 'onions', 'pineapple']
print('mushrooms' in requested_toppings)
print('pepperoni' in requested_toppings)
print('mushrooms' not in requested_toppings)
print('pepperoni' not in requested_toppings)
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| True
False
False
True
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if statements
if statement
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| if conditional_test:
do something
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| age = 19
if age >= 18:
print("You are old enough to vote!")
print("Have you registered to vote yet?")
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| You are old enough to vote!
Have you registered to vote yet?
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if-else statements
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| age = 17
if age >= 18:
print("You are old enough to vote!")
print("Have you registered to vote yet?")
else:
print("Sorry, you are too young to vote.")
print("Please register to vote as soon as you turn 18!")
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| Sorry, you are too young to vote.
Please register to vote as soon as you turn 18!
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if-elif-else chain
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| age = 12
if age < 4:
price = 0
elif age < 18:
price = 25
elif age < 65:
price = 40
else:
price = 20
print(f"Your admission cost is ${price}.")
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| Your admission cost is $25.
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Omit the else block
Python does not require an else block at the end of an if-elif chain. Sometimes an else block is useful; sometimes it is clearer to use an additional elif statement that catches the specific condition of interest:
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| age = 12
if age < 4:
price = 0
elif age < 18:
price = 25
elif age < 65:
price = 40
elif age >= 65:
price = 20
print(f"Your admission cost is ${price}.")
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| Your admission cost is $25.
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Test multiple conditions one by one
Check all of the conditions of interest one by one:
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| requested_toppings = ['mushrooms', 'extra cheese']
if 'mushrooms' in requested_toppings:
print("Adding mushrooms.")
if 'pepperoni' in requested_toppings:
print("Adding pepperoni.")
if 'extra cheese' in requested_toppings:
print("Adding extra cheese.")
print("\nFinished making your pizza!")
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| Adding mushrooms.
Adding extra cheese.
Finished making your pizza!
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Above three if statements are independent.
Use if statements with lists
Check special item
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| requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']
for requested_topping in requested_toppings:
if requested_topping == 'green peppers':
print("Sorry, we are out of green peppers right now.")
else:
print(f"Adding {requested_topping}.")
print("\nFinished making your pizza!")
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| Adding mushrooms.
Sorry, we are out of green peppers right now.
Adding extra cheese.
Finished making your pizza!
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Check whether or not a list is empty
Check whether a list is empty or not before running a for loop:
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| requested_toppings = []
if requested_toppings:
for requested_topping in requested_toppings:
print(f"Adding {requested_topping}.")
print("\nFinished making your pizza!")
else:
print("Are you sure you want a plain pizza?")
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| Are you sure you want a plain pizza?
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Use multiple lists
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| available_toppings = ['mushrooms', 'olives', 'green peppers',
'pepperoni', 'pineapple', 'extra cheese']
requested_toppings = ['mushrooms', 'french fries', 'extra cheese']
for requested_topping in requested_toppings:
if requested_topping in available_toppings:
print(f"Adding {requested_topping}.")
else:
print(f"Sorry, we don't have {requested_topping}.")
print("\nFinished making your pizza!")
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| Adding mushrooms.
Sorry, we don't have french fries.
Adding extra cheese.
Finished making your pizza!
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We can use data structure tuple if the pizzeria has a stable selection of toppings.
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| available_toppings = ('mushrooms', 'olives', 'green peppers',
'pepperoni', 'pineapple', 'extra cheese')
requested_toppings = ['mushrooms', 'french fries', 'extra cheese']
for requested_topping in requested_toppings:
if requested_topping in available_toppings:
print(f"Adding {requested_topping}.")
else:
print(f"Sorry, we don't have {requested_topping}.")
print("\nFinished making your pizza!")
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| Adding mushrooms.
Sorry, we don't have french fries.
Adding extra cheese.
Finished making your pizza!
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Style if statements
The only recommendation PEP 8 provides for styling conditional tests is to use a single space around comparison operators: for example, if age < 4: is better than if age<4:. Which point can be found in “Whitespace in Expressions and Statements” part of PEP 8 documentation.
References