Python code checker
Paste your code and press Check: Python’s own parser finds syntax errors with the exact line and column, and a scan flags names that are never defined. Run shows runtime errors; Fix the error asks AI for a corrected version.
def greet(name)
print(f"Hello, {name}")
greet("Ada")
The most common Python errors and their fixes
| Message | Usual cause | Fix |
|---|---|---|
SyntaxError: expected ':' | Missing colon after def, if, for, while or class | Add : at the end of the line |
SyntaxError: '[' was never closed | Unclosed bracket, often reported on the next line | Close the bracket on the line named in the message |
IndentationError: unindent does not match any outer indentation level | A line indented by a different amount than its block | Use 4 spaces per level, consistently |
NameError: name 'x' is not defined | Typo, or a variable used before it is assigned | Check spelling; assign before use |
TypeError: can only concatenate str (not "int") to str | "a" + 1 | Use an f-string: f"a{1}" |
IndexError: list index out of range | Index ≥ len(list) | Loop with for item in items or check the length |
The messages above are the wording used by Python 3.12 and later, which this checker runs (Python 3.14). Older versions say “invalid syntax” for several of them.
Want to see why a working program gives the wrong answer? Step through it in the Python visualizer. For syntax reminders, keep the Python cheat sheet open.
Frequently asked questions
How do I check Python code for errors online?
Paste your code into the editor above and press Check. It parses the code with the real Python 3 compiler and reports the first syntax error with its line and column, then looks for names that are used but never defined (likely NameErrors). Press Run to execute it and see runtime errors with a full traceback.
What does the checker catch?
Syntax errors (missing colons, unclosed brackets or quotes, bad indentation, invalid assignments) exactly as Python reports them, plus names that are read but never assigned, imported or defined, which usually means a typo. It does not run your code, so it cannot catch errors that depend on values, such as dividing by zero.
Is the checker the same as a linter like pyflakes or pylint?
It is a lightweight subset. The syntax check is exact (it is CPython’s own parser); the undefined-name check is a simple scan and can miss names created dynamically. For a full lint, run pyflakes or ruff locally.
Can AI fix the error for me?
Yes. After Check or Run reports an error, press Fix the error under the editor. Your code and the error message are sent to the AI, which explains the problem and offers corrected code you can drop back into the editor.
Is my code uploaded?
No. Checking and running happen in your browser. Code is only sent to an AI model if you press one of the AI buttons.