Decompression APIs


class zstandard.ZstdDecompressor(dict_data=None, max_window_size=0, format=0)

Context for performing zstandard decompression.

Each instance is essentially a wrapper around a ZSTD_DCtx from zstd’s C API.

An instance can compress data various ways. Instances can be used multiple times.

The interface of this class is very similar to zstandard.ZstdCompressor (by design).

Unless specified otherwise, assume that no two methods of ZstdDecompressor instances can be called from multiple Python threads simultaneously. In other words, assume instances are not thread safe unless stated otherwise.

  • dict_data – Compression dictionary to use.
  • max_window_size – Sets an upper limit on the window size for decompression operations in kibibytes. This setting can be used to prevent large memory allocations for inputs using large compression windows.
  • format

    Set the format of data for the decoder.

    By default this is zstandard.FORMAT_ZSTD1. It can be set to zstandard.FORMAT_ZSTD1_MAGICLESS to allow decoding frames without the 4 byte magic header. Not all decompression APIs support this mode.

copy_stream(ifh, ofh, read_size=131075, write_size=131072)

Copy data between streams, decompressing in the process.

Compressed data will be read from ifh, decompressed, and written to ofh.

>>> dctx = zstandard.ZstdDecompressor()
>>> dctx.copy_stream(ifh, ofh)

e.g. to decompress a file to another file:

>>> dctx = zstandard.ZstdDecompressor()
>>> with open(input_path, 'rb') as ifh, open(output_path, 'wb') as ofh:
...     dctx.copy_stream(ifh, ofh)

The size of chunks being read() and write() from and to the streams can be specified:

>>> dctx = zstandard.ZstdDecompressor()
>>> dctx.copy_stream(ifh, ofh, read_size=8192, write_size=16384)
  • ifh

    Source stream to read compressed data from.

    Must have a read() method.

  • ofh

    Destination stream to write uncompressed data to.

    Must have a write() method.

  • read_size – The number of bytes to read() from the source in a single operation.
  • write_size – The number of bytes to write() to the destination in a single operation.

2-tuple of integers representing the number of bytes read and written, respectively.

decompress(data, max_output_size=0)

Decompress data in its entirety in a single operation.

This method will decompress the entirety of the argument and return the result.

The input bytes are expected to contain a full Zstandard frame (something compressed with ZstdCompressor.compress() or similar). If the input does not contain a full frame, an exception will be raised.

If the frame header of the compressed data does not contain the content size max_output_size must be specified or ZstdError will be raised. An allocation of size max_output_size will be performed and an attempt will be made to perform decompression into that buffer. If the buffer is too small or cannot be allocated, ZstdError will be raised. The buffer will be resized if it is too large.

Uncompressed data could be much larger than compressed data. As a result, calling this function could result in a very large memory allocation being performed to hold the uncompressed data. This could potentially result in MemoryError or system memory swapping. Therefore it is highly recommended to use a streaming decompression method instead of this one.

>>> dctx = zstandard.ZstdDecompressor()
>>> decompressed = dctx.decompress(data)

If the compressed data doesn’t have its content size embedded within it, decompression can be attempted by specifying the max_output_size argument:

>>> dctx = zstandard.ZstdDecompressor()
>>> uncompressed = dctx.decompress(data, max_output_size=1048576)

Ideally, max_output_size will be identical to the decompressed output size.


If the exact size of decompressed data is unknown (not passed in explicitly and not stored in the zstd frame), for performance reasons it is encouraged to use a streaming API.

  • data – Compressed data to decompress.
  • max_output_size

    Integer max size of response.

    If 0, there is no limit and we can attempt to allocate an output buffer of infinite size.


bytes representing decompressed output.


Decompress a series of frames using the content dictionary chaining technique.

Such a list of frames is produced by compressing discrete inputs where each non-initial input is compressed with a prefix dictionary consisting of the content of the previous input.

For example, say you have the following inputs:

>>> inputs = [b"input 1", b"input 2", b"input 3"]

The zstd frame chain consists of:

  1. b"input 1" compressed in standalone/discrete mode
  2. b"input 2" compressed using b"input 1" as a prefix dictionary
  3. b"input 3" compressed using b"input 2" as a prefix dictionary

Each zstd frame must have the content size written.

The following Python code can be used to produce a prefix dictionary chain:

>>> def make_chain(inputs):
...    frames = []
...    # First frame is compressed in standalone/discrete mode.
...    zctx = zstandard.ZstdCompressor()
...    frames.append(zctx.compress(inputs[0]))
...    # Subsequent frames use the previous fulltext as a prefix dictionary
...    for i, raw in enumerate(inputs[1:]):
...        dict_data = zstandard.ZstdCompressionDict(
...            inputs[i], dict_type=zstandard.DICT_TYPE_RAWCONTENT)
...        zctx = zstandard.ZstdCompressor(dict_data=dict_data)
...        frames.append(zctx.compress(raw))
...    return frames

decompress_content_dict_chain() returns the uncompressed data of the last element in the input chain.


It is possible to implement prefix dictionary chain decompression on top of other APIs. However, this function will likely be faster - especially for long input chains - as it avoids the overhead of instantiating and passing around intermediate objects between multiple functions.

Parameters:frames – List of bytes holding compressed zstd frames.

Obtain a standard library compatible incremental decompressor.

See ZstdDecompressionObj for more documentation and usage examples.


Size of decompression context, in bytes.

>>> dctx = zstandard.ZstdDecompressor()
>>> size = dctx.memory_size()
multi_decompress_to_buffer(frames, decompressed_sizes=None, threads=0)

Decompress multiple zstd frames to output buffers as a single operation.

(Experimental. Not available in CFFI backend.)

Compressed frames can be passed to the function as a BufferWithSegments, a BufferWithSegmentsCollection, or as a list containing objects that conform to the buffer protocol. For best performance, pass a BufferWithSegmentsCollection or a BufferWithSegments, as minimal input validation will be done for that type. If calling from Python (as opposed to C), constructing one of these instances may add overhead cancelling out the performance overhead of validation for list inputs.

Returns a BufferWithSegmentsCollection containing the decompressed data. All decompressed data is allocated in a single memory buffer. The BufferWithSegments instance tracks which objects are at which offsets and their respective lengths.

>>> dctx = zstandard.ZstdDecompressor()
>>> results = dctx.multi_decompress_to_buffer([b'...', b'...'])

The decompressed size of each frame MUST be discoverable. It can either be embedded within the zstd frame or passed in via the decompressed_sizes argument.

The decompressed_sizes argument is an object conforming to the buffer protocol which holds an array of 64-bit unsigned integers in the machine’s native format defining the decompressed sizes of each frame. If this argument is passed, it avoids having to scan each frame for its decompressed size. This frame scanning can add noticeable overhead in some scenarios.

>>> frames = [...]
>>> sizes = struct.pack('=QQQQ', len0, len1, len2, len3)
>>> dctx = zstandard.ZstdDecompressor()
>>> results = dctx.multi_decompress_to_buffer(frames, decompressed_sizes=sizes)


It is possible to pass a mmap.mmap() instance into this function by wrapping it with a BufferWithSegments instance (which will define the offsets of frames within the memory mapped region).

This function is logically equivalent to performing ZstdCompressor.decompress() on each input frame and returning the result.

This function exists to perform decompression on multiple frames as fast as possible by having as little overhead as possible. Since decompression is performed as a single operation and since the decompressed output is stored in a single buffer, extra memory allocations, Python objects, and Python function calls are avoided. This is ideal for scenarios where callers know up front that they need to access data for multiple frames, such as when delta chains are being used.

Currently, the implementation always spawns multiple threads when requested, even if the amount of work to do is small. In the future, it will be smarter about avoiding threads and their associated overhead when the amount of work to do is small.

  • frames – Source defining zstd frames to decompress.
  • decompressed_sizes – Array of integers representing sizes of decompressed zstd frames.
  • threads

    How many threads to use for decompression operations.

    Negative values will use the same number of threads as logical CPUs on the machine. Values 0 or 1 use a single thread.



read_to_iter(reader, read_size=131075, write_size=131072, skip_bytes=0)

Read compressed data to an iterator of uncompressed chunks.

This method will read data from reader, feed it to a decompressor, and emit bytes chunks representing the decompressed result.

>>> dctx = zstandard.ZstdDecompressor()
>>> for chunk in dctx.read_to_iter(fh):
...     # Do something with original data.

read_to_iter() accepts an object with a read(size) method that will return compressed bytes or an object conforming to the buffer protocol.

read_to_iter() returns an iterator whose elements are chunks of the decompressed data.

The size of requested read() from the source can be specified:

>>> dctx = zstandard.ZstdDecompressor()
>>> for chunk in dctx.read_to_iter(fh, read_size=16384):
...    pass

It is also possible to skip leading bytes in the input data:

>>> dctx = zstandard.ZstdDecompressor()
>>> for chunk in dctx.read_to_iter(fh, skip_bytes=1):
...    pass


Skipping leading bytes is useful if the source data contains extra header data. Traditionally, you would need to create a slice or memoryview of the data you want to decompress. This would create overhead. It is more efficient to pass the offset into this API.

Similarly to ZstdCompressor.read_to_iter(), the consumer of the iterator controls when data is decompressed. If the iterator isn’t consumed, decompression is put on hold.

When read_to_iter() is passed an object conforming to the buffer protocol, the behavior may seem similar to what occurs when the simple decompression API is used. However, this API works when the decompressed size is unknown. Furthermore, if feeding large inputs, the decompressor will work in chunks instead of performing a single operation.

  • reader – Source of compressed data. Can be any object with a read(size) method or any object conforming to the buffer protocol.
  • read_size – Integer size of data chunks to read from reader and feed into the decompressor.
  • write_size – Integer size of data chunks to emit from iterator.
  • skip_bytes – Integer number of bytes to skip over before sending data into the decompressor.

Iterator of bytes representing uncompressed data.

stream_reader(source, read_size=131075, read_across_frames=False, closefd=True)

Read-only stream wrapper that performs decompression.

This method obtains an object that conforms to the io.RawIOBase interface and performs transparent decompression via read() operations. Source data is obtained by calling read() on a source stream or object implementing the buffer protocol.

See zstandard.ZstdDecompressionReader for more documentation and usage examples.

  • source – Source of compressed data to decompress. Can be any object with a read(size) method or that conforms to the buffer protocol.
  • read_size – Integer number of bytes to read from the source and feed into the compressor at a time.
  • read_across_frames – Whether to read data across multiple zstd frames. If False, decompression is stopped at frame boundaries.
  • closefd – Whether to close the source stream when this instance is closed.


stream_writer(writer, write_size=131072, write_return_read=True, closefd=True)

Push-based stream wrapper that performs decompression.

This method constructs a stream wrapper that conforms to the io.RawIOBase interface and performs transparent decompression when writing to a wrapper stream.

See zstandard.ZstdDecompressionWriter for more documentation and usage examples.

  • writer – Destination for decompressed output. Can be any object with a write(data).
  • write_size – Integer size of chunks to write() to writer.
  • write_return_read – Whether write() should return the number of bytes of input consumed. If False, write() returns the number of bytes sent to the inner stream.
  • closefd – Whether to close() the inner stream when this stream is closed.



class zstandard.ZstdDecompressionWriter(decompressor, writer, write_size, write_return_read, closefd=True)

Write-only stream wrapper that performs decompression.

This type provides a writable stream that performs decompression and writes decompressed data to another stream.

This type implements the io.RawIOBase interface. Only methods that involve writing will do useful things.

Behavior is similar to ZstdCompressor.stream_writer(): compressed data is sent to the decompressor by calling write(data) and decompressed output is written to the inner stream by calling its write(data) method:

>>> dctx = zstandard.ZstdDecompressor()
>>> decompressor = dctx.stream_writer(fh)
>>> # Will call fh.write() with uncompressed data.
>>> decompressor.write(compressed_data)

Instances can be used as context managers. However, context managers add no extra special behavior other than automatically calling close() when they exit.

Calling close() will mark the stream as closed and subsequent I/O operations will raise ValueError (per the documented behavior of io.RawIOBase). close() will also call close() on the underlying stream if such a method exists and the instance was created with closefd=True.

The size of chunks to write() to the destination can be specified:

>>> dctx = zstandard.ZstdDecompressor()
>>> with dctx.stream_writer(fh, write_size=16384) as decompressor:
>>>    pass

You can see how much memory is being used by the decompressor:

>>> dctx = zstandard.ZstdDecompressor()
>>> with dctx.stream_writer(fh) as decompressor:
>>>    byte_size = decompressor.memory_size()

stream_writer() accepts a write_return_read boolean argument to control the return value of write(). When True (the default)``, write() returns the number of bytes that were read from the input. When False, write() returns the number of bytes that were write() to the inner stream.

seek(offset, whence=None)


class zstandard.ZstdDecompressionReader(decompressor, source, read_size, read_across_frames, closefd=True)

Read only decompressor that pull uncompressed data from another stream.

This type provides a read-only stream interface for performing transparent decompression from another stream or data source. It conforms to the io.RawIOBase interface. Only methods relevant to reading are implemented.

>>> with open(path, 'rb') as fh:
>>> dctx = zstandard.ZstdDecompressor()
>>> reader = dctx.stream_reader(fh)
>>> while True:
...     chunk =
...     if not chunk:
...         break
...     # Do something with decompressed chunk.

The stream can also be used as a context manager:

>>> with open(path, 'rb') as fh:
...     dctx = zstandard.ZstdDecompressor()
...     with dctx.stream_reader(fh) as reader:
...         ...

When used as a context manager, the stream is closed and the underlying resources are released when the context manager exits. Future operations against the stream will fail.

The source argument to stream_reader() can be any object with a read(size) method or any object implementing the buffer protocol.

If the source is a stream, you can specify how large read() requests to that stream should be via the read_size argument. It defaults to zstandard.DECOMPRESSION_RECOMMENDED_INPUT_SIZE.:

>>> with open(path, 'rb') as fh:
...     dctx = zstandard.ZstdDecompressor()
...     # Will perform when obtaining data for the decompressor.
...     with dctx.stream_reader(fh, read_size=8192) as reader:
...         ...

Instances are partially seekable. Absolute and relative positions (SEEK_SET and SEEK_CUR) forward of the current position are allowed. Offsets behind the current read position and offsets relative to the end of stream are not allowed and will raise ValueError if attempted.

tell() returns the number of decompressed bytes read so far.

Not all I/O methods are implemented. Notably missing is support for readline(), readlines(), and linewise iteration support. This is because streams operate on binary data - not text data. If you want to convert decompressed output to text, you can chain an io.TextIOWrapper to the stream:

>>> with open(path, 'rb') as fh:
...     dctx = zstandard.ZstdDecompressor()
...     stream_reader = dctx.stream_reader(fh)
...     text_stream = io.TextIOWrapper(stream_reader, encoding='utf-8')
...     for line in text_stream:
...         ...
seek(pos, whence=0)


class zstandard.ZstdDecompressionObj(decompressor, write_size)

A standard library API compatible decompressor.

This type implements a compressor that conforms to the API by other decompressors in Python’s standard library. e.g. zlib.decompressobj or bz2.BZ2Decompressor. This allows callers to use zstd compression while conforming to a similar API.

Compressed data chunks are fed into decompress(data) and uncompressed output (or an empty bytes) is returned. Output from subsequent calls needs to be concatenated to reassemble the full decompressed byte sequence.

Each instance is single use: once an input frame is decoded, decompress() can no longer be called.

>>> dctx = zstandard.ZstdDecompressor()
>>> dobj = dctx.decompressobj()
>>> data = dobj.decompress(compressed_chunk_0)
>>> data = dobj.decompress(compressed_chunk_1)

By default, calls to decompress() write output data in chunks of size DECOMPRESSION_RECOMMENDED_OUTPUT_SIZE. These chunks are concatenated before being returned to the caller. It is possible to define the size of these temporary chunks by passing write_size to decompressobj():

>>> dctx = zstandard.ZstdDecompressor()
>>> dobj = dctx.decompressobj(write_size=1048576)


Because calls to decompress() may need to perform multiple memory (re)allocations, this streaming decompression API isn’t as efficient as other APIs.


Send compressed data to the decompressor and obtain decompressed data.

Parameters:data – Data to feed into the decompressor.
Returns:Decompressed bytes.

Effectively a no-op.

Implemented for compatibility with the standard library APIs.

Safe to call at any time.

Returns:Empty bytes.