Brython-specific built-in modules
Working with Brython
Frequently asked questions
Q : what does "Brython" mean ?
A : Browser Python. It's also the Welsh word for
Q : which browsers support Brython ?
A : all modern browsers, including on smartphones. The generated
Note that performance is usually better (sometimes much better) with
Firefox than with Chrome.
Q : what is the performance of Brython compared to CPython ?
R : this page compares the execution time of a
number of elementary operations between the latest version of Brython on
Firefox and CPython. The results vary depending on operations, but overall
the order of magnitude is the same.
The Brython repository includes a script, at address localhost:8000/speed,
that compares the speed of Brython and CPython on the local machine for a
variety of elementary operations.
the Brython site, it can be used
equivalent in Python in the editor (unchecking the "debug" box)
The difference is due to two factors :
Q : I see a lot of 404 errors in the browser console when I run Brython
scripts, why is that ?
A : this is because of the way Brython implements the "import" mechanism.
When a script has to import module X, Brython searches for a file or a package
modules, Lib for Python modules), the directory Lib/site-packages, the
directory of current page. For that, Ajax calls are sent to the matching urls;
if the file is not found, the 404 error message is written in the browser
console, but the error is caught by Brython which goes on searching until it
finds the module, or raises
give an idea, the module datetime (2130 lines of Python code) is parsed and
specifications of Python, including the dynamic nature of the search
ImportError if all paths have been tried with no
Q : why does this message show in the browser console : "Synchronous
XMLHttpRequest on the main thread is deprecated because of its detrimental
effects to the end user's experience. For more help
A : this is also related to imports, or to file reading. To achieve these
operations, Brython uses blocking Ajax calls : an imported module must be
loaded before it can be used. Browser vendors should normally not remove
blocking calls any time soon.
Q : is it possible to precompile Brython scripts, in order to reduce
execution time ?
code in a
<script> section of an HTML page, load the page, edit the code,
reload the page, etc. It is not like other projects where the Python code is
have to run this script before reloading the page.
Another reason why it is a not a good idea to precompile Brython is that the
generated code is typically 10 times bigger than the original Python source -
this is the price to pay for compliance with the language specification. The
page would take longer to load, and we haven't found that this would be faster
than compiling on the fly.
However, since version 3.6.0, a precompiled version of the scripts in the
standard library is stored in an indexedDB database attached to the browser
where the code is executed. The compilation is performed the first time a
script is imported, or if the Brython version changed since the last
compilation. This improves dramatically the imports loading time.
Q : I am trying to import a module from the Brython standard distribution
and I get an error message, why ?
R : the most likely reason is that the script brython_stdlib.js
has not been included in the page.
Q : can I import all the modules / packages that run with CPython ?
R : no, only those that are completely written in Python. The programs
that use extensions written in C are not supported. For instance, Numpy,
Matplotlib, Pandas cannot work with Brython.
It is the same for modules / packages that call primitives of the operating
system that are not available in the browser context: for instance requests,
which uses the IP stack to execute HTTP requests to arbitrary urls, when a
browser can only send Ajax requests to the same domain, or the the (rare)
sites that support cross-origin requests.
Q : why use the operator
<= to build the tree of DOM elements ? This
is not pythonic !
A : Python has no built-in structure to manipulate trees, ie to add
"child" or "sibling" nodes to a tree node. For these operations, functions
can be used ; the syntax proposed by Brython is to use operators : this is
easier to type (no parenthesis) and more readable
To add a sibling node, the operator
+ is used.
To add a child, the operator
<= was chosen for these reasons :
Anyway, those who are allergic to operator overloading can use the method
- it has the shape of a left arrow ; note that Python function annotations use
a new operator
-> that was chosen for its arrow shape
- it can't be confused with "lesser or equal" because a line with
document <= elt would be a no-op if it was "lesser or equal", which is
always used in a condition or as the return value of a function
- in Python,
<= is used as an operator for sets with a different meaning
than "lesser or equal"
- Python uses the same operator
% for very different operations : modulo
and string formatting
pathlib module in Python standard library uses the operator
to build file system paths
attach() of DOM elements instead.