If-Statement Pitfalls

Note

Source: Adapted from the C# edition (decisions/ifpitfalls.rst). Python-specific pitfalls (truthiness testing, is None, missing elif) are original additions.

As you start writing if statements, watch out for these common mistakes.

Using = Instead of ==

In Python, = is assignment and == is comparison. Using = inside a condition is a SyntaxError:

# SyntaxError in Python (unlike C, which silently allows this)
if x = 5:
    print("five")

# correct
if x == 5:
    print("five")

Python’s syntax rule prevents this entire class of bug.

Indentation Errors

Python uses indentation to define blocks. Mixing tabs and spaces, or using inconsistent indentation, causes an IndentationError or TabError:

# wrong — second print is not part of the if body
if x > 0:
    print("positive")
print("This always runs")   # outside the if

# correct — both prints inside the body
if x > 0:
    print("positive")
    print("This also only runs when x > 0")

A good code editor highlights indentation and warns about inconsistencies.

Missing elif (Redundant Checks)

When cases are mutually exclusive, use elif rather than separate if statements. Using separate if statements checks all conditions even after one has matched:

# wrong — all three ifs are checked even when the first matches
score = 95
if score >= 90:
    grade = "A"
if score >= 80:    # this also runs! grade gets overwritten to "B"
    grade = "B"
if score >= 70:
    grade = "C"

# correct — only the first matching branch runs
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"

Testing Truthiness

Python allows any value in a condition — not just a true Boolean. This can be convenient but also a source of confusion:

name = input("Enter name: ")

# works but potentially misleading
if name:
    print("Name provided.")

# explicit — clearer intent
if name != "":
    print("Name provided.")

For None checks, always use is (not ==):

result = some_function()

if result is None:        # correct
    print("No result.")

if result == None:        # works but not idiomatic
    print("No result.")

if not result:            # WRONG if 0, [], or "" are valid results
    print("No result.")

Off-by-One in Range Conditions

When checking membership in a range, be precise about < vs <=:

# should include 100 as a valid score, but doesn't
if 0 <= score < 100:
    print("valid")

# correct
if 0 <= score <= 100:
    print("valid")

Always test the boundary values: exactly 0 and exactly 100 in this case.

Forgetting else

If a variable is set inside if but not in an else, it may be uninitialized when the condition is false:

if score >= 60:
    status = "pass"
# if score < 60, status is never set — NameError below!
print(status)

Fix: always set the variable before the if, or add an else:

status = "fail"          # default
if score >= 60:
    status = "pass"
print(status)