10 Common Python Pitfalls and How to Avoid Them

1. Avoid mutable default arguments

Using a list as a default parameter makes it act like a global variable, causing unexpected accumulation across calls.

def foo(num, age=[]):
    age.append(num)
    print("num", num)
    return age
print(foo(1))
print(foo(2))
print(foo(3))

The above prints:

num 1
[1]
num 2
[1, 2]
num 3
[1, 2, 3]

Fix by using None as the default and creating a new list inside the function.

def foo(num, age=None):
    if not age:
        age = []
    age.append(num)
    print("num", num)
    return age
print(foo(1))
print(foo(2))
print(foo(3))

2. Using for‑else to find primes

The for‑else construct runs the else block only when the loop completes without a break, which is handy for prime detection.

# find primes up to 100
import math
num = []
for i in range(2, 100):
    for j in range(2, int(math.sqrt(i)) + 1):
        if i % j == 0:
            break
    else:
        num.append(i)
for index, i in enumerate(num):
    if index == len(num) - 1:
        print(i)
    else:
        print(i, end=" ")

This demonstrates why the final else is essential for correctly identifying primes.

3. Dictionary key hashing quirks

Dictionary keys are compared by equality and hash value; 1 and 1.0 share the same hash, so the later assignment overwrites the former.

a = {}
a[1] = "A"
a[1.0] = "B"
a[2] = "C"
print(a)

Result: {1: 'B', 2: 'C'} Hashes of immutable objects with the same value are equal, e.g.:

hash(1) == hash(1.0) == hash(True)
hash(0) == hash(False) == hash("")

4. Proper if‑condition checks

In Python, empty containers evaluate to False, so if not a: is preferred over a == None.

a = []
b = {}
c = ""
if not a:
    print("a not empty")
if not b:
    print("b not empty")
if not c:
    print("c not empty")

For many branches, a dispatch dictionary is cleaner:

my_dict = {"A": 1, "B": 2}
print(my_dict[type])

When initializing many attributes from a dict, use self.__dict__.update(dicts):

class A():
    def __init__(self, dicts):
        self.__dict__.update(dicts)
        print(self.__dict__)

if __name__ == '__main__':
    dicts = {"name": "lisa", "age": 23, "sex": "women", "hobby": "hardstyle"}
    a = A(dicts)

5. Closure lazy‑evaluation trap

Lambda functions capture variables by reference, so all lambdas see the final loop value.

ls = []
for x in range(5):
    ls.append(lambda: x**2)
print(ls[0]())
print(ls[1]())
print(ls[2]())

All three calls output 16 because x is 4 after the loop ends.

6. File path vs current working directory

Use os.getcwd() for the current working directory and sys.path[0] for the script’s directory.

import os
os.getcwd()
import sys
sys.path[0]

7. eval cannot parse numbers with a leading zero

eval("02")

This raises SyntaxError: invalid token because leading zeros are not allowed in numeric literals.

8. while 1 vs while True

In Python 2, True could be reassigned, making while 1 slightly faster; in Python 3, True is a keyword, so both loops behave identically.

9. Handling long multiline strings

Wrap a triple‑quoted string in parentheses to avoid accidental line‑break issues:

("""multi line text""")

10. requests encoding shortcut

Set the response encoding to the detected one with a single line:

res.encoding = res.apparent_encoding