Frequently asked questions

Syntax, keywords and built-in functions

Standard distribution

import implementation

Browser interface

Brython-specific built-in modules

Working with Brython

Brython for Firefox OS



Brython implements Python version 3, based on the Python Language Reference

The implementation takes into account the browsers limitations, in particular those related to the file system. Writing is impossible, and reading is limited to the folders accessible with an Ajax request.

Keywords and built-in functions

Brython supports most of the keywords and functions of Python 3 :

  • keywords : as, assert, break, class, continue, def, del, elif, else, except, False, finally, for, from, global, if, import, is, lambda, None, nonlocal, pass, return, True, try, while, with, yield
  • built-in functions and classes : abs, all, any, ascii, bin, bool, bytes, callable, chr, classmethod, delattr, dict, dir, divmod, enumerate, eval, exec, filter, float, frozenset, getattr, globals, hasattr, hash, hex, id, input, int, isinstance, iter, len, list, locals, map, max, memoryview, min, next, object, open, ord, pow, print, property, range, repr, reversed, round, set, setattr, slice, sorted, str, ` sum, super, tuple, type, vars, zip, __import__

Here are a few features and limitations imposed by the browser and Javascript :

  • the built-in function open() takes as argument the url of the file to open. Since it is read with an Ajax call, it must be in the same domain as the script. The object returned by open() has the usual reading and access methods : read, readlines, seek, tell, close

  • by default, print() will output to the web browser console and so are the error messages. sys.stderr and sys.stdout can be assigned to an object with a write() method, and this allows for the redirection of output to go to a window or text area, for example.

  • to open a print dialog (to a printer), call window.print (window is defined in module browser).

  • sys.stdin is not implemented at this time, however there is an input() built-in function that will open a blocking input dialog (a prompt).

  • the objects lifecycle is managed by the Javascript garbage collector, Brython doesn't manage reference counting like CPython. Therefore, method __del__() is not called when a class instance is no more referenced.

  • functions such as time.sleep() that block execution during a given time, or until an event is triggered, are not managed because there is no Javascript equivalent. In this case, the application must be written with the functions of module browser.timer (eg set_timeout(), set_interval()), or by event handlers (method bind() of DOM elements).

  • the JSON parser uses that of Javascript ; because of that, the real numbers that are equal to integers (eg 1.0) are converted into integers by json.dumps().

Built-in value __name__

The built-in variable __name__ is set to the value of the attribute id of the script. For instance:

<script type="text/python" id="myscript">
assert __name__ == 'myscript'

If 2 scripts have the same id, an exception is raised.

For scripts that don't have an explicit id set :

  • if no script has its id set to __main__, the first "unnamed" script has its __name__ set to "__main__". So, if there only one script in the page, it will be able to run the usual test :

<script type="text/python">
if __name__=='__main__':
    print('hello !')

  • for the other unnamed scripts, __name__ is set to a random string starting with __main__