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A very lightweight, memory-efficient async web framework for Pycopy ( and its uasyncio module.

Project description

picoweb is a “micro” web micro-framework (thus, “pico-framework”) for radically unbloated web applications using radically unbloated Python implementation, Pycopy, .


  • Asynchronous from the start, using unbloated asyncio-like library for Pycopy (uasyncio). This means that picoweb can process multiple concurrent requests at the same time (using I/O and/or CPU multiplexing).

  • Small memory usage. Initial version required about 64K of heap for a trivial web app, and since then, it was optimized to allow run more or less realistic web app in ~36K of heap. More optimizations on all the levels (Pycopy and up) are planned (but may lead to API changes).

  • Has API affinity with some well-known Python web micro-framework(s), thus it should be an easy start if you have experience with them, and existing applications can be potentially ported, instead of requiring complete rewrite.

Requirements and optional modules

picoweb depends on uasyncio for asynchronous networking ( uasyncio itself requires Pycopy <>, a minimalist, lightweight, and resource-efficient Python language implementation.

It is also indended to be used with utemplate ( for templating, but this is a “soft” dependency - picoweb offers convenience functions to use utemplate templates, but if you don’t use them or will handle templating in your app (e.g. with a different library), it won’t be imported.

For database access, there are following options (picoweb does not depend on any of them, up to your application to choose):

  • btree builtin Pycopy module. This is a recommended way to do a database storage for picoweb, as it allows portability across all Pycopy targets, starting with very memory- and storage-limited baremetal systems.

  • btreedb wrapper on top of btree builtin module. This may add some overhead, but may allow to make an application portable between different database backends (filedb and uorm below).

  • filedb, for a simple database using files in a filesystem

  • uorm, for Sqlite3 database access (works only with Pycopy Unix port)

Last but not least, picoweb uses a standard logging-compatible logger for diagnostic output (like a connection opened, errors and debug information). However this output is optional, and otherwise you can use a custom logging class instead of the standard logging/ulogging module. Due to this, and to not put additional dependencies burden on the small webapps for small systems, logging module is not included in picoweb’s installation dependencies. Instead, a particular app using picoweb should depend on pycopy-ulogging or pycopy-logging package. Note that to disable use of logging, an application should start up using The default value for debug parameter is 0 however, in which case picoweb will use ulogging module (on which your application needs to depend, again).


picoweb API is roughly based on APIs of other well-known Python web frameworks. The strongest affinity is Flask,, as arguably the most popular micro-framework. Some features are also based on Bottle and Django. Note that this does not mean particular “compatibility” with Flask, Bottle, or Django: most existing web frameworks are synchronous (and threaded), while picoweb is async framework, so its architecture is quite different. However, there is an aim to save porting efforts from repetitive search & replace trials: for example, when methods do similar things, they are likely named the same (but they may take slightly different parameters, return different values, and behave slightly differently).

The biggest difference is async, non-threaded nature of picoweb. That means that the same code may handle multiple requests at the same time, but unlike threaded environment, there’s no external context (like thread and thread local storage) to associate with each request. Thus, there’re no “global” (or thread-local “global”) request and response objects, like Flask, Bottle, Django have. Instead, all picoweb functions explicitly pass the current request and response objects around.

Also, picoweb, being unbloated framework, tries to avoid avoidable abstractions. For example, HTTP at the lowest level has just read and write endpoints of a socket. To dispatch request, picoweb needs to pre-parse some request data from input stream, and it saves that partially (sic!) parsed data as a “request” object, and that’s what passed to application handlers. However, there’s no unavoidable need to have a “response” abstraction - the most efficient/lightweight application may want to just write raw HTTP status line, headers, and body to the socket. Thus, raw write stream is passed to application handlers as the “response” object. (But high-level convenience functions to construct an HTTP response are provided).

API reference

The best API reference currently are examples (see below) and the picoweb source code itself. It’s under 10K, so enjoy:

Note that API is experimental and may undergo changes.


  • - A simple webapp showing you how to generate a complete HTTP response yourself, use picoweb convenience functions for HTTP headers generation, and use of templates. Mapping from URLs to webapp view functions (“web routes” or just “routes”) is done Django-style, using a centralized route list.

  • - Like above, but uses app.route() decorator for route specification, Flask-style.

  • examples/ - Additional examples for various features of picoweb. See comments in each file for additional info. To run examples in this directory, you normally would need to have picoweb installed (i.e. available in your MICROPYPATH, which defaults to ~/.micropython/lib/).

  • notes-pico - A more realistic example webapp, ported from the Flask original.

Running under CPython (regressed)

Initial versions of picoweb could run under CPython, but later it was further optimized for Pycopy, and ability to run under CPython regressed. It’s still on TODO to fix it, instructions below tell how it used to work.

At least CPython 3.4.2 is required (for asyncio loop.create_task() support). To run under CPython, uasyncio compatibility module for CPython is required (pycopy-cpython-uasyncio). This and other dependencies can be installed using requirements-cpython.txt:

pip install -r requirements-cpython.txt

Reporting Issues

Here are a few guidelines to make feedback more productive:

  1. Please be considerate of the overall best practices and common pitfalls in reporting issues, this document gives a good overview: How to Report Bugs Effectively.

  2. The reference platform for picoweb is the Unix port of Pycopy. All issues reported must be validated against this version, to differentiate issues of picoweb/uasyncio from the issues of your underlying platform.

  3. All reports must include version information of all components involved: Pycopy, picoweb, uasyncio, uasyncio.core, any additional modules. Generally, only the latest versions of the above are supported (this is what you get when you install/reinstall components using the upip package manager). The version information are thus first of all important for yourself, the issue reporter, it allows you to double-check if you’re using an outdated or unsupported component.

  4. Feature requests: picoweb is by definition a pico-framework, and bound to stay so. Feature requests are welcome, but please be considerate that they may be outside the scope of core project. There’s an easy way out though: instead of putting more stuff into picoweb, build new things on top of it: via plugins, subclassing, additional modules etc. That’s how it was intended to be from the beginning!

  5. We would like to establish a dedicated QA team to support users of this project better. If you would like to sponsor this effort, please let us know.

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