10 Common Python Mistakes Every Developer Should Avoid

Python is a powerful and flexible language, but developers often fall into subtle pitfalls that can cause confusing bugs. This guide summarizes ten common Python mistakes and shows how to avoid them.

Common Mistake 1: Using an expression as a function's default argument

When a default value is a mutable type, it is evaluated only once at function definition time, causing the same object to be reused across calls.

Calling foo() repeatedly appends to the same list instead of creating a new one.

Solution: use None as the default and create a new list inside the function.

Common Mistake 2: Misusing class variables

Class variables are shared across all instances and subclasses. Accessing an undefined attribute in a subclass falls back to the base class, which can lead to unexpected behavior.

Common Mistake 3: Incorrect exception block parameters

In Python 2, the except clause must bind the exception to a variable using as. Using an identifier after except without as does not catch the intended exception.

Correct form:

except (IndexError, KeyError) as e:

Common Mistake 4: Misunderstanding LEGB variable resolution

Python follows the LEGB rule (Local, Enclosing, Global, Built‑in). Assigning to a variable inside a function makes it local, which can shadow variables from outer scopes and cause UnboundLocalError.

Common Mistake 5: Modifying a list while iterating over it

Removing elements from a list during iteration leads to skipped items or index errors. Use a list comprehension or iterate over a copy instead.

Common Mistake 6: Late binding in closures

Closures capture variables by reference, so all generated functions may see the final loop value. Use default arguments to bind the current value.

Common Mistake 7: Circular module dependencies

Importing modules that reference each other can cause AttributeError if a name is accessed before it is defined. Delay the import or restructure code to break the cycle.

Common Mistake 8: Naming conflicts with standard library modules

Creating a module with the same name as a standard‑library module (e.g., email.py) can cause the interpreter to import the wrong module, leading to hard‑to‑diagnose errors.

Common Mistake 9: Ignoring Python 2 vs Python 3 differences

Exception objects are not accessible outside the except block in Python 3. Keep a reference to the exception if you need it later.

Common Mistake 10: Misusing the del method

When a module is cleaned up, its globals are set to None before __del__ runs, causing AttributeError. Register cleanup functions with atexit.register() to ensure proper finalization.

By understanding and avoiding these ten pitfalls, Python developers can write cleaner, more reliable code and prevent subtle bugs that often arise from language nuances.