Run a Python script or work at a REPL with the standard library to hand, output as it is written and a panel of what the run left bound. No server and no install.
How it works
Scripts run on Pyodide, the CPython interpreter compiled to WebAssembly, in a worker of the page's own. The interpreter and its standard library, about 13 MB, are served from this site and fetched on the first Run, which is why that one says “Starting Python…” for a moment; later runs reuse them.
Each Run executes the script in a fresh set of globals, so the last run's names are gone, although modules it imported stay loaded. Standard output and standard error are caught as the bytes are written, so a print with end="" appears at once and the two streams interleave in the order they were written. A traceback starts at the first frame of your own script, with the page's machinery left out.
When the run ends, the panel beside the editor lists every name it left bound, with its type and a preview. Dicts, lists, sets and instances open into their contents, and names that came from an import wait under a closed Imported heading. Previews are cut short as they are built, and a container opens to its first 100 items and a count of the rest.
The REPL uses the same interpreter and keeps one namespace between entries. A trailing expression is printed with its repr and bound to _. A compound statement such as for or if waits for a blank line, so an else can follow, and lines added with Shift+Enter run as one entry.
A loop that prints fast is paced: the page redraws at most every 100 ms, keeps the last 128 KB, and says when earlier output was dropped. The JavaScript page is the same console with a JavaScript engine behind it.
Examples
A traceback that opens on your line
def mean(values):
return sum(values) / len(values)
print(mean([3, 4, 5]))
print(mean([]))4.0
Traceback (most recent call last):
File "<script>", line 5, in <module>
File "<script>", line 2, in mean
ZeroDivisionError: division by zeroThe first print ran before the error, so its output stays. Both frames are the script's own, and the panel still lists mean, which was bound before line 5 raised.
What the panel shows after a run
from collections import Counter
words = "the cat sat on the mat".split()
counts = Counter(words)
print(counts.most_common(2))[('the', 2), ('cat', 1)]The panel then reads:
words list (6) ['the', 'cat', 'sat', 'on', 'the', 'mat']
counts Counter (5) Counter({'the': 2, 'cat': 1, 'sat': 1, 'on': 1, 'mat': 1})
Imported 1 name
Counter type <class 'collections.Counter'>counts opens into its five keys. Counter is filed under Imported because the script's own import bound it, which the page reads from the code rather than guessing from the value.
A block at the prompt
>>> total = 0
>>> for n in range(1, 5):
... total += n
...
>>> total
10
>>> _ * 2
20The for line and its body wait until the blank line closes the block. total is answered with its repr, and _ holds that value for the line after.
Common problems
ModuleNotFoundError: No module named 'numpy'
Only the standard library is there. numpy, pandas, requests and everything else from PyPI are missing, and nothing on the page installs them: micropip would fetch from a package index, and this page fetches nothing but the interpreter.
RuntimeError: can't start new thread
WebAssembly in a browser gives the interpreter no threads and no processes. threading and socket import but cannot work, Thread.start() raises that error, subprocess.run raises OSError: [Errno 138] emscripten does not support processes, and multiprocessing fails to import.
ZoneInfoNotFoundError for a zone name
ZoneInfo("Europe/London") needs a time zone database, which Pyodide keeps in a separate tzdata package this page does not load. datetime.timezone.utc and fixed offsets such as timezone(timedelta(hours=2)) work.
input() raises OSError: [Errno 29] I/O error
The page gives the interpreter no standard input, so input() fails instead of waiting for a reply. Put the values in the script, or assign them at the REPL.
Frequently asked questions
Which version of Python is it?
Whichever CPython the bundled Pyodide release is built on. The sample script the page opens with prints it, and so does import sys; print(sys.version).
Do variables carry over from one Run to the next?
No. Each Run starts on empty globals, so a script cannot lean on something an earlier one defined; modules it imported are still loaded, which only makes importing them again fast. The REPL is the mode that keeps names, until Stop ends the interpreter.
How do I stop a script that never ends?
Press Stop. It terminates the worker the interpreter runs in, which is the one thing that can interrupt while True:, and whatever the script had printed stays on screen. The next Run or REPL line starts a new interpreter, so a REPL session's names are gone.