Online Python Compiler
Write Python 3 and run it instantly, right in your browser. Feed input() from the stdin box, import numpy, pandas or matplotlib, click a traceback to jump to the failing line, and share your program with a link.
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Python 3 runs on your device with Pyodide (CPython compiled to WebAssembly). The first run downloads the runtime (about 10 MB, cached afterwards); imports such as numpy and pandas are loaded automatically. Your code is never uploaded.
How to use the Online Python Compiler
- Write or paste Python in the editor, open a .py file, or pick an example.
- Add input in the Input (stdin) box if your program calls input(): one line per call.
- Press Run code or Ctrl + Enter. The first run downloads the Python runtime; later runs start instantly.
- Fix errors by clicking the line link in a traceback to jump to that line in the editor.
- Share or save your program with Copy share link or Download.
How this Python compiler works
There is no server behind the Run button. The page loads Pyodide, a build of the official CPython interpreter compiled to WebAssembly, inside a Web Worker. Your program is compiled to Python bytecode and executed by that interpreter on your own CPU, which is why output appears without network round trips and why your code stays private. Each run starts with a fresh __main__ namespace, so variables from a previous run do not leak into the next one, while packages you already loaded stay in memory and import instantly.
Two things are handled differently from a terminal: input() reads from the Input (stdin) box instead of waiting for keystrokes, and plt.show() turns each open matplotlib figure into an image in the output panel.
Browser Python vs. desktop Python
| Feature | In this compiler | Tip |
|---|---|---|
| Standard library (math, re, json, datetime, collections, itertools, dataclasses, decimal, fractions) | Works | Same behavior as CPython 3.12 |
input() and sys.stdin | Reads the stdin box | One line per input() call |
| numpy, pandas, scipy, sympy, scikit-learn, matplotlib | Loaded on import | First import downloads the package (a few MB) |
open() files | In-memory file system | Files vanish when Python restarts |
time.sleep() | Works, but blocks the run | Counts toward the time limit |
| threading, multiprocessing, subprocess | Not available | Use plain loops or asyncio |
| requests, sockets, urllib to other sites | Not available | Browsers do not allow raw sockets |
| tkinter, pygame windows, turtle | Not available | Draw charts with matplotlib instead |
Reading a Python traceback
A traceback lists the calls that led to an error, oldest first. Read it from the bottom up: the last line names the exception type and message, for example KeyError: 'grace', and the File "main.py", line N entry just above it is where Python stopped. Common ones: NameError (a variable or function name is misspelled or used before it is defined), IndentationError (mixed tabs and spaces or a missing indent after a colon), TypeError (for example adding a string to a number: convert with int() or str()), IndexError and KeyError (a list position or dictionary key that does not exist) and EOFError (your program asked for more input than the stdin box contains).
Frequently asked questions
Is Python compiled or interpreted?
CPython, the standard Python implementation, compiles your source code to bytecode and then interprets that bytecode in a virtual machine. This tool runs the same CPython, itself compiled to WebAssembly by the Pyodide project, so your code executes inside your browser rather than on a remote server.
How do I use input() in this online compiler?
Type the values your program reads into the Input (stdin) box, one line for each input() call, before you press Run. Each call takes the next line and the value is echoed after the prompt, like in a terminal. If the box runs out of lines, input() raises EOFError. sys.stdin.read() returns the whole box.
Can I use numpy, pandas and matplotlib?
Yes. When your code imports a package that Pyodide provides, such as numpy, pandas, scipy, sympy, scikit-learn or matplotlib, it is downloaded and loaded automatically before your program runs. matplotlib figures are drawn as PNG images below the output. Packages with C extensions that Pyodide does not build, and anything needing network sockets or threads, are not available.
Which Python version does it run?
Pyodide 0.26 includes CPython 3.12, so f-strings, match statements, dataclasses, type hints and the rest of modern Python work. The exact version is shown in the status bar once the runtime has loaded.
What happens if my code has an infinite loop?
Python runs in a separate Web Worker, so the page stays responsive. When a run exceeds the time limit you chose (10 seconds by default), or you press Stop, the worker is shut down and Python restarts on the next run. Use a higher limit for long calculations.
Can my program read files or access the internet?
Your program has a private in-memory file system, so open() can write and read files during a run, but it cannot see files on your computer; open a .py file into the editor instead. Raw sockets, subprocess and threads are not available in the browser, so libraries such as requests do not work.
Is my code private, and how do share links work?
Code runs entirely on your device and is never uploaded. A share link stores your code and stdin, compressed, after the # in the URL; that part is not sent to any server. Opening a shared link loads the code but does not run it until you press Run.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.