Common Python errors and troubleshooting guide

Anatomy of a Python traceback

When the CPython 3.12 interpreter encounters an unhandled exception, it formats an execution traceback across standard error. The traceback prints chronologically from the initial call frame down to the specific line that triggered the crash.

The final line names the exception class (ZeroDivisionError) and provides descriptive text explaining the failure condition. The preceding lines indicate that the failure happened inside compute_average on line 14 of main.py.

Ten frequent Python errors and their solutions

These ten exception types account for the majority of execution bugs in Python scripts.

1. SyntaxError: invalid syntax

The interpreter raises a SyntaxError during the parsing stage before any code executes. This occurs when you omit a colon after an if statement or leave a parenthesis open.

2. IndentationError: unexpected indent

Python uses whitespace indentation to delimit code blocks instead of curly braces. Mixing tabs and spaces triggers an IndentationError. Set your text editor to insert 4 spaces when pressing the Tab key.

3. NameError: name 'variable' is not defined

A NameError indicates that your code referenced an identifier that does not exist in local or global scope. Common causes include misspelled variable names or referencing a variable before its assignment statement.

4. TypeError: can only concatenate str (not "int") to str

Python does not perform implicit string coercion when using the addition operator. You must explicitly convert numbers with str() or use Python 3.6 f-strings.

5. ValueError: invalid literal for int() with base 10

This exception occurs when passing arbitrary strings into numerical conversion functions such as int() or float(). Validate input with str.isdigit() or enclose the conversion inside a try-except block.

6. IndexError: list index out of range

Python lists use zero-based indexing. A list with 5 elements contains indices from 0 through 4. Requesting index 5 raises an IndexError.

7. KeyError: 'key_name'

Directly indexing a dictionary with square brackets raises a KeyError if the key is missing. Use the dict.get() method with a fallback default value to avoid exceptions.

8. ZeroDivisionError: division by zero

Dividing a number by zero or performing a modulo operation by zero triggers this exception. Always test divisors for non-zero values before arithmetic operations.

9. ModuleNotFoundError: No module named 'requests'

This exception occurs when your script imports a third-party library that is not present in sys.path. In Python Code Runner, open the Packages tab and install the library via micropip. On Windows, execute pip install requests in PowerShell.

10. AttributeError: 'NoneType' object has no attribute 'append'

This error happens when you assign the return value of an in-place list method to a variable. In Python, methods like list.sort() and list.append() mutate the list in place and return None.

Frequently asked questions about online Python execution