Python in the browser

Event handlers, markup expressions, computed values and helpers that browser code calls are compiled from Python to JavaScript. PyWeb compiles a defined subset of Python, with Python's semantics where JavaScript differs, and reports anything outside it as a compile error with the file and line.

The translation is checked continuously by a differential test that evaluates well over a hundred expressions in CPython and in the compiled JavaScript and requires identical results, including which exception is raised.

#Supported

Expressions: literals (int, float, str, bool, None, list, tuple, dict, set), f-strings (with format specs such as {x:.2f}, {n:,}, {v!r}), arithmetic (+ - * / // % **, with Python's floor division and modulo signs), comparisons including chained ones (0 < x < 10), == on lists and dicts (structural), in/not in, is None, and/or/not with Python truthiness (empty list, dict and string are false), conditional expressions, indexing with negative indices, slicing with steps, list/dict/set comprehensions and generator expressions (multiple for/if clauses), lambdas, unpacking in literals ([*a, *b], {**d}), await.

Statements: assignment (including tuple unpacking and chained assignment), augmented assignment, if/elif/else, for (with tuple targets), while, break, continue, return, pass, del x[k], try/except/finally (matching on exception class names, except X as e), raise, assert, nested def.

Builtins: len, str, repr, int, float, bool, abs, min, max (with key=, default=), sum, round (banker's rounding like Python), range, sorted (key=, reverse=), reversed, enumerate, zip, list, tuple, dict, set, any, all, chr, ord, isinstance (with builtin type names), print (to the browser console).

Methods:

• str: upper lower strip lstrip rstrip split splitlines join replace startswith endswith find rfind index count title capitalize isdigit isnumeric isalpha isalnum isspace isupper islower zfill ljust rjust center format
• list: append extend insert pop remove index count clear sort reverse copy
• dict: get keys values items pop update setdefault copy clear
• set: add discard remove clear copy

Errors: out-of-range indexes raise IndexError, missing keys KeyError, bad conversions ValueError, mixing str + int TypeError, division by zero ZeroDivisionError, so the same try/except you would write in Python works.

#Not supported (compile errors)

Classes, with, import inside functions, generators (yield), global state outside the page, the walrus operator, *args/**kwargs parameters, slice assignment, for/else and while/else, keyword arguments to arbitrary JavaScript functions, integers larger than 2**53, and any module-level name that only exists on the server (imports, database handles, classes). For those, call an @server function.

#Known differences

• JavaScript has one number type: 2.0 prints as 2 and 1/3 uses float arithmetic as in Python, but int and float are not distinguished when printing whole numbers.
• dict values in the browser are plain JSON objects, so keys are strings. Attribute access (user.name) and subscripting (user["name"]) both work on them.
• print writes to the browser console.
• Strings compare by UTF-16 code units, which only differs from Python for characters outside the Basic Multilingual Plane.

#Seeing the output

The compiled module for each page is readable JavaScript (minified only by pyweb build --production). In pyweb dev open /static/<PageName>.js to see exactly what runs in the browser.