Truthy and falsy values in Python

Introduction

Python uses truthy and falsy values to decide if something is true or false in conditions, making decisions easy and natural.

Checking if a list has any items before processing it.

Deciding if a string is empty or has content.

Testing if a number is zero or not before calculations.

Controlling flow in if statements without writing explicit comparisons.

Simplifying code by relying on Python's built-in truthiness rules.

Syntax

Python

if value:
    # runs if value is truthy
else:
    # runs if value is falsy

In Python, values like 0, None, empty collections ([], {}, '', ()), and False are falsy.

All other values are considered truthy.

Examples

Zero is falsy, so the else block runs.

Python

if 0:
    print("This won't print")
else:
    print("Zero is falsy")

Non-empty strings are truthy, so this prints.

Python

if "hello":
    print("Non-empty string is truthy")

Empty lists are falsy, so else block runs.

Python

if []:
    print("This won't print")
else:
    print("Empty list is falsy")

None is falsy, so else block runs.

Python

if None:
    print("This won't print")
else:
    print("None is falsy")

Sample Program

This program checks each value in the list and prints if it is truthy or falsy.

Python

values = [0, 1, "", "text", [], [1,2], None, True, False]

for v in values:
    if v:
        print(f"{repr(v)} is truthy")
    else:
        print(f"{repr(v)} is falsy")

Important Notes

Remember that empty containers like lists, tuples, sets, and dictionaries are falsy.

Custom objects are truthy by default unless they define __bool__ or __len__ methods.

Using truthy/falsy values can make your code shorter and easier to read.

Summary

Python treats certain values as true or false automatically in conditions.

Falsy values include 0, None, False, and empty containers like '', [], (), and {}.

All other values are truthy and make conditions run the 'if' block.

Practice

1. Which of the following values is considered falsy in Python?

easy

A. An empty list []

B. A non-empty string 'hello'

C. The integer 5

D. A non-empty dictionary {'key': 'value'}

Solution

1. Step 1: Understand falsy values in Python

   Falsy values are those that behave like False in conditions. Common falsy values include None, 0, empty sequences like [], '', and empty collections like {}.

2. Step 2: Check each option

   An empty list [] is an empty list, which is falsy. Options B, C, and D are non-empty and thus truthy.

3. Final Answer:

   An empty list [] -> Option A

4. Quick Check:

   Empty list is falsy = A [OK]

Hint: Empty containers and zero are falsy, others are truthy [OK]

Common Mistakes:

• Thinking non-empty collections are falsy
• Confusing zero with non-zero numbers
• Assuming all strings are falsy

2. Which of the following is the correct way to check if a variable x is falsy in Python?

easy

A. if not x:

B. if x == False:

C. if x is False:

D. if x = False:

Solution

1. Step 1: Understand Python falsy check syntax

   To check if a value is falsy, use if not x: which tests if x behaves like False in a boolean context.

2. Step 2: Analyze each option

   if x == False: compares x to False but misses other falsy values like 0 or ''. if x is False: checks identity with False, which is too strict. if x = False: has a syntax error (= instead of ==). if not x: is the correct idiomatic way.

3. Final Answer:

   if not x: -> Option A

4. Quick Check:

   Use if not x: to check falsy [OK]

Hint: Use 'if not x:' to test falsy values in Python [OK]

Common Mistakes:

• Using assignment '=' instead of comparison '=='
• Checking identity with 'is False' instead of truthiness
• Comparing directly to False misses other falsy values

3. What will be the output of the following code?

values = [0, 1, '', 'Python', [], [1, 2]]
result = [bool(v) for v in values]
print(result)

medium

A. [True, True, True, True, True, True]

B. [False, True, False, True, False, True]

C. [False, False, False, False, False, False]

D. [True, False, True, False, True, False]

Solution

1. Step 1: Evaluate boolean value of each element

   0, empty string '', and empty list [] are falsy, so their bool() is False. Non-zero numbers, non-empty strings, and non-empty lists are truthy, so bool() is True.

2. Step 2: Map each value to bool()

   values = [0(False), 1(True), ''(False), 'Python'(True), [] (False), [1, 2](True)]

3. Final Answer:

   [False, True, False, True, False, True] -> Option B

4. Quick Check:

   Falsy are 0, '', [] = False [OK]

Hint: Empty or zero values are False, others True in bool() [OK]

Common Mistakes:

• Assuming all numbers are True
• Thinking empty strings are True
• Confusing empty and non-empty lists

4. The following code is intended to print "Falsy" if val is falsy, but it always prints "Truthy". What is the error?

val = 0
if val == False:
    print("Falsy")
else:
    print("Truthy")

medium

A. Using '==' instead of 'is' for comparison

B. The variable 'val' should be converted to bool first

C. Comparing with False misses some falsy values

D. No error; the code works correctly

Solution

1. Step 1: Understand comparison with False

   Using val == False only matches if val equals exactly False or behaves equal to it. But some falsy values like 0 compare equal to False, so this seems correct here.

2. Step 2: Check why it prints "Truthy"

   Actually, 0 == False is True, so it should print "Falsy". If it prints "Truthy", likely the code is different or the question expects the explanation that comparing with False is not reliable for all falsy values like empty containers or None.

3. Final Answer:

   Comparing with False misses some falsy values -> Option C

4. Quick Check:

   Use 'if not val:' to catch all falsy [OK]

Hint: Use 'if not val:' to catch all falsy values, not '== False' [OK]

Common Mistakes:

• Thinking '==' always works for falsy check
• Using 'is' instead of '==' incorrectly
• Not realizing empty containers are falsy but not equal to False

5. You want to filter out all falsy values from a list data = [0, 1, '', 'text', [], [1], None]. Which code snippet correctly creates a new list with only truthy values?

hard

A. filtered = [x for x in data if x is not False]

B. filtered = [x for x in data if x == True]

C. filtered = [x for x in data if bool(x) == False]

D. filtered = [x for x in data if x]

Solution

1. Step 1: Understand filtering truthy values

   To keep only truthy values, we filter with if x which keeps values that behave like True.

2. Step 2: Analyze each option

   filtered = [x for x in data if x] uses if x, which is correct. filtered = [x for x in data if x == True] checks x == True, which excludes truthy but not exactly True values (like 1). filtered = [x for x in data if bool(x) == False] keeps falsy values (bool(x) == False). filtered = [x for x in data if x is not False] excludes only False but keeps other falsy values like 0 or ''.

3. Final Answer:

   filtered = [x for x in data if x] -> Option D

4. Quick Check:

   Use 'if x' to filter truthy values [OK]

Hint: Use list comprehension with 'if x' to keep truthy values [OK]

Common Mistakes:

• Using '== True' excludes some truthy values
• Filtering with 'bool(x) == False' keeps falsy values
• Using 'is not False' misses other falsy values