python-gotchas

Quick Reference

Gotcha	Problem	Fix
Mutable default	def f(x=[])	Use None, create in function
Iterate + mutate	Skips items	Iterate over copy items[:]
is vs ==	Identity vs value	Use is only for None
Late binding	lambda: i captures var	lambda i=i: i
Float precision	0.1 + 0.2 != 0.3	math.isclose()
Dict mutation	RuntimeError	list(d.keys())
Class attribute	Shared mutable	Init in __init__

Falsy Values	Examples
Boolean	False
None	None
Numbers	0, 0.0, 0j
Empty collections	"", [], {}, set()

Scope Rule	Order
LEGB	Local → Enclosing → Global → Built-in
global	Access module-level variable
nonlocal	Access enclosing function variable

When to Use This Skill

Use for debugging and prevention:

• Understanding why code behaves unexpectedly
• Avoiding common Python pitfalls
• Reviewing code for subtle bugs
• Learning Python's evaluation rules
• Fixing mutable default arguments

Related skills:

• For fundamentals: see python-fundamentals-313
• For testing: see python-testing
• For type hints: see python-type-hints

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Python Common Gotchas and Pitfalls

Overview

Python has several well-known pitfalls that trip up developers of all experience levels. Understanding these gotchas prevents subtle bugs and unexpected behavior.

1. Mutable Default Arguments

The Problem

# BAD: Mutable default argument
def add_item(item, items=[]):
    items.append(item)
    return items

# Unexpected behavior!
print(add_item("a"))  # ['a']
print(add_item("b"))  # ['a', 'b'] - NOT ['b']!
print(add_item("c"))  # ['a', 'b', 'c']

Why It Happens

Default arguments are evaluated once when the function is defined, not each time it's called. The same list object is reused across all calls.

The Fix

# GOOD: Use None as default
def add_item(item, items=None):
    if items is None:
        items = []
    items.append(item)
    return items

# Works correctly
print(add_item("a"))  # ['a']
print(add_item("b"))  # ['b']
print(add_item("c"))  # ['c']

Other Mutable Defaults

# BAD: All mutable types have this issue
def bad_dict(data={}): ...
def bad_set(data=set()): ...
def bad_class(config=SomeClass()): ...

# GOOD: Always use None
def good_dict(data=None):
    if data is None:
        data = {}
    return data

def good_set(data=None):
    if data is None:
        data = set()
    return data

2. Mutating Lists While Iterating

The Problem

# BAD: Modifying list during iteration
numbers = [1, 2, 3, 4, 5, 6]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)

print(numbers)  # [1, 3, 5] - missed 4!

Why It Happens

The iterator uses indices internally. When you remove an item, all subsequent indices shift, causing items to be skipped.

The Fixes

# GOOD: Iterate over a copy
numbers = [1, 2, 3, 4, 5, 6]
for num in numbers[:]:  # Slice creates a copy
    if num % 2 == 0:
        numbers.remove(num)
print(numbers)  # [1, 3, 5]

# GOOD: Use list comprehension (preferred)
numbers = [1, 2, 3, 4, 5, 6]
numbers = [num for num in numbers if num % 2 != 0]
print(numbers)  # [1, 3, 5]

# GOOD: Use filter
numbers = [1, 2, 3, 4, 5, 6]
numbers = list(filter(lambda x: x % 2 != 0, numbers))
print(numbers)  # [1, 3, 5]

# GOOD: Iterate backwards (for in-place modification)
numbers = [1, 2, 3, 4, 5, 6]
for i in range(len(numbers) - 1, -1, -1):
    if numbers[i] % 2 == 0:
        del numbers[i]
print(numbers)  # [1, 3, 5]

3. is vs ==

The Problem

# Comparing values vs identity
a = [1, 2, 3]
b = [1, 2, 3]

print(a == b)  # True - same values
print(a is b)  # False - different objects

# Integer interning gotcha
x = 256
y = 256
print(x is y)  # True (integers -5 to 256 are interned)

x = 257
y = 257
print(x is y)  # False! (outside interning range)

The Rule

• Use == to compare values
• Use is only for identity (singletons like None, True, False)

# GOOD: Correct usage
if value is None:
    ...

if value == other_value:
    ...

# BAD: Don't use `is` for value comparison
if value is 0:  # Wrong!
    ...

4. Variable Scope (LEGB)

The Problem

# Closure gotcha
functions = []
for i in range(3):
    functions.append(lambda: i)

# All return the same value!
print([f() for f in functions])  # [2, 2, 2]

Why It Happens

The lambda captures the variable i, not its value. By the time lambdas are called, i is 2.

The Fixes

# GOOD: Capture value with default argument
functions = []
for i in range(3):
    functions.append(lambda i=i: i)  # Default arg captures value
print([f() for f in functions])  # [0, 1, 2]

# GOOD: Use functools.partial
from functools import partial

def return_value(x):
    return x

functions = [partial(return_value, i) for i in range(3)]
print([f() for f in functions])  # [0, 1, 2]

UnboundLocalError

# BAD: This raises UnboundLocalError
x = 10

def increment():
    x = x + 1  # Error! x is local but used before assignment
    return x

# GOOD: Use global (sparingly)
def increment():
    global x
    x = x + 1
    return x

# BETTER: Avoid global, pass as parameter
def increment(x):
    return x + 1

5. String Concatenation

Implicit Concatenation Gotcha

# Missing comma creates concatenation
items = [
    "apple"
    "banana"  # Oops! Missing comma
    "cherry"
]
print(items)  # ['applebanana', 'cherry']

# CORRECT
items = [
    "apple",
    "banana",
    "cherry",
]

Type Mixing

# BAD: Can't concatenate str and int
name = "User"
count = 42
# message = "Hello " + name + ", you have " + count + " messages"  # TypeError!

# GOOD: Use f-strings
message = f"Hello {name}, you have {count} messages"

# GOOD: Use str()
message = "Hello " + name + ", you have " + str(count) + " messages"

6. Late Binding in Closures

The Problem

# Class method gotcha
class MyClass:
    def __init__(self, callbacks=[]):  # BAD: Mutable default!
        self.callbacks = callbacks

    def add_callback(self, func):
        self.callbacks.append(func)

obj1 = MyClass()
obj2 = MyClass()
obj1.add_callback(lambda: print("Hello"))

# obj2 also has the callback!
print(len(obj2.callbacks))  # 1

The Fix

class MyClass:
    def __init__(self, callbacks=None):
        self.callbacks = callbacks if callbacks is not None else []

    def add_callback(self, func):
        self.callbacks.append(func)

7. Boolean Evaluation

Falsy Values

# These are all falsy
falsy_values = [
    False,
    None,
    0,
    0.0,
    0j,
    "",
    [],
    {},
    set(),
    range(0),
]

# Gotcha: Empty collections are falsy
data = []
if data:
    print("Has data")  # Not printed
else:
    print("No data")   # Printed

# But None and empty are different!
if data is None:
    print("Is None")      # Not printed
elif data == []:
    print("Is empty list")  # Printed

Explicit Checks

# BAD: Ambiguous check
def process(items):
    if not items:  # Could be None OR empty
        return

# GOOD: Be explicit about what you're checking
def process(items):
    if items is None:
        raise ValueError("items cannot be None")
    if len(items) == 0:
        return  # Early return for empty list

8. Floating Point Precision

The Problem

# Floating point arithmetic isn't exact
print(0.1 + 0.2)  # 0.30000000000000004
print(0.1 + 0.2 == 0.3)  # False!

The Fixes

import math
from decimal import Decimal

# GOOD: Use math.isclose for comparisons
print(math.isclose(0.1 + 0.2, 0.3))  # True

# GOOD: Use Decimal for financial calculations
price = Decimal("19.99")
tax = Decimal("0.0875")
total = price * (1 + tax)
print(total)  # 21.739125

# Round appropriately
print(round(total, 2))  # 21.74

9. Exception Handling

Bare Except

# BAD: Catches everything, including KeyboardInterrupt
try:
    risky_operation()
except:
    pass

# BAD: Too broad
try:
    risky_operation()
except Exception:
    pass  # Silently ignores all errors

# GOOD: Catch specific exceptions
try:
    risky_operation()
except ValueError as e:
    logger.error(f"Invalid value: {e}")
except ConnectionError as e:
    logger.error(f"Connection failed: {e}")
    raise  # Re-raise after logging

Exception Variable Scope

# Python 3: Exception variable is deleted after except block
try:
    1 / 0
except ZeroDivisionError as e:
    error = e  # Save if needed
    print(e)

# print(e)  # NameError! e is deleted
print(error)  # OK

10. Dictionary Key Ordering

Modern Python (3.7+)

# Since Python 3.7, dicts maintain insertion order
d = {"b": 2, "a": 1, "c": 3}
print(list(d.keys()))  # ['b', 'a', 'c'] - insertion order

# But comparison ignores order
d1 = {"a": 1, "b": 2}
d2 = {"b": 2, "a": 1}
print(d1 == d2)  # True

Gotcha: Changing Dict During Iteration

# BAD: RuntimeError
d = {"a": 1, "b": 2, "c": 3}
for key in d:
    if d[key] == 2:
        del d[key]  # RuntimeError: dictionary changed size during iteration

# GOOD: Iterate over copy of keys
d = {"a": 1, "b": 2, "c": 3}
for key in list(d.keys()):
    if d[key] == 2:
        del d[key]
print(d)  # {'a': 1, 'c': 3}

# GOOD: Dict comprehension
d = {"a": 1, "b": 2, "c": 3}
d = {k: v for k, v in d.items() if v != 2}

11. Import Gotchas

Circular Imports

# module_a.py
from module_b import func_b  # Fails if module_b imports from module_a

def func_a():
    return func_b()

# SOLUTION: Import inside function
def func_a():
    from module_b import func_b
    return func_b()

# OR: Import module, not function
import module_b

def func_a():
    return module_b.func_b()

Module Name Shadowing

# BAD: File named random.py shadows stdlib
# random.py (your file)
import random  # Imports YOUR file, not stdlib!
random.randint(1, 10)  # AttributeError

# Python 3.13 now warns about this!

12. Class Attribute vs Instance Attribute

class MyClass:
    shared_list = []  # Class attribute - shared by all instances!

    def add_item(self, item):
        self.shared_list.append(item)

a = MyClass()
b = MyClass()
a.add_item("hello")
print(b.shared_list)  # ['hello'] - Oops!

# GOOD: Initialize in __init__
class MyClass:
    def __init__(self):
        self.items = []  # Instance attribute - unique to each instance

    def add_item(self, item):
        self.items.append(item)

Quick Reference

Gotcha	Problem	Solution
Mutable defaults	def f(x=[])	Use None, create in function
Mutating while iterating	Skips items	Iterate over copy or use comprehension
is vs ==	Identity vs equality	Use is only for None, True, False
Late binding	Captures variable, not value	Use default argument to capture
Falsy values	Empty != None	Be explicit in checks
Float precision	0.1 + 0.2 != 0.3	Use math.isclose() or Decimal
Bare except	Catches too much	Catch specific exceptions
Dict iteration	Can't modify during	Iterate over list(d.keys())
Circular imports	Import errors	Import inside function or import module
Class attributes	Shared unexpectedly	Initialize in __init__