Streaming multipart/form-data parser

streaming_form_data provides a Python parser for parsing multipart/form-data input chunks (the most commonly used encoding when submitting data through HTML forms).

Chunk size is determined by the API user, but currently there are no restrictions on what the chunk size should be, since the parser works byte-by-byte (which means that passing the entire input as a single chunk should also work).


$ pip install streaming_form_data

The core parser is written in Cython, which is a superset of Python that compiles the input down to a C extension which can then be imported in normal Python code.

The compiled C parser code is included in the PyPI package, hence the installation requires a working C compiler.


>>> from streaming_form_data import StreamingFormDataParser
>>> from streaming_form_data.targets import ValueTarget, FileTarget, NullTarget
>>> headers = {'Content-Type': 'multipart/form-data; boundary=boundary'}
>>> parser = StreamingFormDataParser(headers=headers)
>>> parser.register('name', ValueTarget())
>>> parser.register('file', FileTarget('/tmp/file.txt'))
>>> parser.register('discard-me', NullTarget())
>>> parser.data_received(chunk)

Usage can broadly be split into three stages.

1. Initialization

The StreamingFormDataParser class expects a dictionary of HTTP request headers when being instantiated. These headers are used to determine the input Content-Type and a few other metadata.

2. Input Registration

HTML forms typically have multiple fields. For instance, a form could have a text input field called name and a file input field called file.

This needs to be communicated to the parser using the parser.register function. This function expects two arguments - the name of the input field, and the associated Target class (which determines how the input should be handled).

For instance, if you want to store the contents of the name field in an in-memory variable, and the file field in a file on disk, you can tell this to the parser as follows.

>>> name_target = ValueTarget()
>>> file_target = FileTarget('/tmp/file.dat')
>>> parser.register('name', name_target)
>>> parser.register('file', file_target)

3. Streaming data

At this stage the parser has everything it needs to be able to work. Depending on what web framework you’re using, just pass the actual HTTP request body to the parser, either one chunk at a time or the complete thing at once.

>> chunk = read_next_chunk() # depends on your web framework of choice
>> parser.data_received(chunk)



This class is the main entry point, and expects a dictionary of HTTP request headers. These headers are used to determine the input Content-Type and a few other metadata.

Target classes

When registering inputs with the parser, instances of subclasses of the Target class should be used. These target classes ultimately determine what to do with the data.


ValueTarget objects hold the input in memory.

>>> target = ValueTarget()


FileTarget objects stream the contents to a file on-disk.

>>> target = FileTarget('/tmp/file.txt')


SHA256Target objects calculate a SHA256 hash of the given input, and hold the result in memory.

>>> target = SHA256Target()


NullTarget objects discard the input completely.

>>> target = NullTarget()

Custom Target classes

It’s possible to define custom targets for your specific use case by inheriting the streaming_form_data.targets.BaseTarget class and overriding the on_data_received function.

>>> from streaming_form_data.targets import BaseTarget
>>> class CustomTarget(BaseTarget):
...     def on_data_received(self, chunk):
...         do_something_with(chunk)

If the Content-Disposition header included the filename directive, this value will be available as the self.multipart_filename attribute in Target classes.

Validator classes

Target classes accept a validator callable when being instantiated. Every time data_received is called with a given chunk, the target runs this chunk through the given callable.

This is useful for performing certain validation tasks like making sure the input size is not exceeding a certain value. This is shown in the following code snippet.

>>> from streaming_form_data.targets import ValueTarget
>>> target = ValueTarget(validator=MaxSizeValidator(100))


If you’d like to document usage with another web framework (which ideally allows chunked HTTP reads), please open an issue or a pull request.

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