What is it?
The Python Checker finds the mistakes that stop a Python program before it starts: IndentationError, a missing colon, an unclosed bracket or quote, and tabs mixed with spaces. It also explains common beginner slips such as = instead of ==, print without brackets, else if instead of elif, ++, and a list used as a default argument. One button compiles the code with real Python 3.12 in your browser for a complete syntax check.
Who is it for?
School and college students, beginners following a course, teachers checking homework quickly, and anyone on a phone or a computer without Python installed.
How to use it
- Paste your code into the Python code to check box, or tap Try a sample to see how it works.
- The list of problems updates as you type: ✕ marks an error, ! a warning. The line above it sums up, for example “2 errors, 1 warning. Tap Show to jump to the line.”
- For a full check, press Check with real Python. The first time it shows “Loading Python (about 10 MB, only the first time)…”; afterwards the button says Check again.
- Tap Show next to a problem to jump to that line, fix it, and watch the list update. Problems that are not on one line say Whole file. Clear empties the box.
Worked example
Paste:
if total = 100 print "Done"
The checker points to three things: = should be == in a comparison, the if line needs a colon, and print needs brackets in Python 3. The fixed code is if total == 100: followed by an indented print("Done").
It only checks the code; it never runs it. Compare with None using “is None”, not “== None”.
Is my code uploaded?
No. The checker is downloaded once from a public download server (jsDelivr or cdnjs) and then runs inside your browser. Your code never leaves your device, so it is safe for work and client projects.
Errors and how to fix them
- “✓ No common mistakes found. Press “Check with real Python” for a full syntax check”: the quick checks passed; press the button to be sure.
- “✓ No problems found, and Python compiles it without errors”: the code is valid Python. Logic mistakes can still exist, so run it in Run Python.
- Mixed tabs and spaces: use 4 spaces for every indent level and never mix them.
- Missing colon: every if, for, while, def and class line ends with :.
- Nothing happens, or a “Could not load” message: the first use downloads the library. Check your internet connection, turn off an ad or script blocker for this site, and reload the page.
Open the tool: Python Checker
Still stuck?
Contact support. Tell us which tool or page you were using, what you tapped, what you saw, and your phone or computer and browser (for example: Android phone, Chrome). We reply by email.
Questions, or an error not listed here?
Tell us in the comments: what you were doing, the exact message you saw, and your device and browser. We read every comment and add new fixes to this article.