7 Python Code Bugs You Need to Fix

June 14, 2026

Jump to Section:

1. Don’t use iterators
2. Don’t use ELSE in a loop
3. Incorrect use of IF-ELSE
4. Return different data types in RETURN
5. Incorrect comparison with TRUE
6. Don’t use ZIP when iterating over lists
7. Use KEY IN LIST
Conclusion: Writing Clean, Pythonic Code

Python is one of the most popular programming languages, trusted by developers for its simplicity, readability, and powerful capabilities. However, as with any language, working with Python can lead to a number of common errors that can not only slow down the development process but also lead to unexpected results.

In this article, we’ll look at common mistakes made by both beginners and experienced programmers and discuss how to avoid them.

1. Don’t use iterators

Almost everyone has made this mistake at one time or another, regardless of their experience with other programming languages.

Typical task: go through a list in a loop for.

The standard approach is to create a counter iand get each element of the list.

The correct approach is to remember that it listis an iterable object.

Right:

for element in list_:

foo(element)

Wrong:

for i in range(len(list_)):

foo(list_[i])

2. Don’t use ELSE in a loop

Python allows you to use a block elsein loops for.

If the loop forterminates without being interrupted by the breakor operators return, then the block will be executed else.

Right:

l = [1, 2, 3]

num = 4

for n in l:
if n == num :
print(“num found”)
break
else:

print("num not found")

Wrong:

l = [1, 2, 3]

num = 4

found = False

for n in l:
if n == num:
found = True
print(“num found”)
breakif not found:
print(“num not found”)

3. Incorrect use of IF-ELSE

When you check some condition inside a function, sometimes there is no need to write a block if-else– sometimes it is enough to just use if.

Right:

def func(a):

if a > 5:

return True

return False

Wrong:

def func(a):

if a > 5:

return True

else:

return False

4. Return different data types in RETURN

Returning different data types in a block returndepending on the situation leads to code complexity and errors. To avoid such situations, throw an exception when an error occurs.

Right:

def get_code(pswrd):

if pswrd != "bicycle":

raise ValueError

else:

return "42"

try:
code = get_code(“car”)
print(f”code is {code}“)
except ValueError:
print(“Wrong pswrd”)

Wrong:

def get_code(pswrd):

if pswrd != "bicycle":

return None

else:

return "42"

code = get_code(“car”)if code is None:

print("Wrong pswrd")

else:

print(f"The code is {code}")

5. Incorrect comparison with TRUE

The best way to check if a variable or expression is True> is to use a pattern if cond: expr. Patterns if cond == True: exprand if cond is True: exprless preferred.

Right:

flag = True

if flag:

print("PEP 8 Style Guide prefers this pattern")

Wrong:

flag = True

if flag == True:

print("This works, but is not the PEP 8 pattern")

6. Don’t use ZIP when iterating over lists

When iterating over multiple lists, you might want to create a counter iand rangeretrieve list elements by index. Don’t do this—use the zip.

Right:

num = [1, 2, 3]

let = ["A", "B", "C"]

for n, l in zip(num, let):

print(n, l)

Wrong:

num = [1, 2, 3]

let = ["A", "B", "C"]

for index in range(len(num)):

print(num[index], let[index])

7. Use KEY IN LIST

If you want to check whether a value appears keyin a list list, don’t blindly use the [] construct key in list, as it’s not always optimal. Use sets—this way, you won’t scan the entire list, but only the unique elements.

Right:

l = [1, 2, 3, 4, 1, 1, 2]

if 3 in set(l):

 print("3 is in the list.")

else:

print("3 is NOT in the list.")

Wrong:

l = [1, 2, 3, 4, 1, 1, 2]

if 3 in l:

print("3 is in the list.")

else:

print("3 is NOT in the list.")

Conclusion: Writing Clean, Pythonic Code

Mastering Python isn’t just about making your code work; it’s about making it readable, efficient, and maintainable. As we’ve seen, many common bugs and inefficiencies stem from bringing habits from other programming languages over to Python, or simply from overlooking the built-in features that make the language so powerful.

By avoiding these 7 common pitfalls, you achieve several critical improvements in your development workflow:

Readability: Leveraging Pythonic structures like direct iterators, zip(), and clean if Statements make your code read like natural English.
Performance: Swapping out brute-force list searches for hash-based lookups like set() can drastically speed up your execution time as your data scales.
Predictability: Enforcing strict return types and utilizing built-in loop structures like for...else reduces hidden edge cases and keeps your debugging sessions short.

The journey to writing better code relies on a willingness to continuously audit your habits against established guidelines like PEP 8. Next time you find yourself reaching for a manual index counter or chaining unnecessary else blocks, take a step back and ask: “Is there a more Pythonic way to do this?” More often than not, the answer will save you time, lines of code, and future headaches.

7 Python Code Bugs You Need to Fix

1. Don’t use iterators

2. Don’t use ELSE in a loop

3. Incorrect use of IF-ELSE

4. Return different data types in RETURN

5. Incorrect comparison with TRUE

6. Don’t use ZIP when iterating over lists

7. Use KEY IN LIST

Conclusion: Writing Clean, Pythonic Code

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