# `compression.zstd` — Compression compatible with the Zstandard format

Added in version 3.14.

Source code: Lib/compression/zstd/__init__.py

This module provides classes and functions for compressing and decompressing data using the Zstandard (or zstd) compression algorithm. The zstd manual describes Zstandard as “a fast lossless compression algorithm, targeting real-time compression scenarios at zlib-level and better compression ratios.” Also included is a file interface that supports reading and writing the contents of `.zst` files created by the zstd utility, as well as raw zstd compressed streams.

The `compression.zstd` module contains:

- The `open()` function and `ZstdFile` class for reading and writing compressed files.

- The `ZstdCompressor` and `ZstdDecompressor` classes for incremental (de)compression.

- The `compress()` and `decompress()` functions for one-shot (de)compression.

- The `train_dict()` and `finalize_dict()` functions and the `ZstdDict` class to train and manage Zstandard dictionaries.

- The `CompressionParameter`, `DecompressionParameter`, and `Strategy` classes for setting advanced (de)compression parameters.

## Exceptions

exception compression.zstd.ZstdError

  This exception is raised when an error occurs during compression or decompression, or while initializing the (de)compressor state.

## Reading and writing compressed files

compression.zstd.open(file, /, mode='rb', *, level=None, options=None, zstd_dict=None, encoding=None, errors=None, newline=None)

  Open a Zstandard-compressed file in binary or text mode, returning a file object.

  The file argument can be either a file name (given as a `str`, `bytes` or path-like object), in which case the named file is opened, or it can be an existing file object to read from or write to.

  The mode argument can be either `'rb'` for reading (default), `'wb'` for overwriting, `'ab'` for appending, or `'xb'` for exclusive creation. These can equivalently be given as `'r'`, `'w'`, `'a'`, and `'x'` respectively. You may also open in text mode with `'rt'`, `'wt'`, `'at'`, and `'xt'` respectively.

  When reading, the options argument can be a dictionary providing advanced decompression parameters; see `DecompressionParameter` for detailed information about supported parameters. The zstd_dict argument is a `ZstdDict` instance to be used during decompression. When reading, if the level argument is not None, a `TypeError` will be raised.

  When writing, the options argument can be a dictionary providing advanced decompression parameters; see `CompressionParameter` for detailed information about supported parameters. The level argument is the compression level to use when writing compressed data. Only one of level or options may be non-None. The zstd_dict argument is a `ZstdDict` instance to be used during compression.

  In binary mode, this function is equivalent to the `ZstdFile` constructor: `ZstdFile(file, mode, ...)`. In this case, the encoding, errors, and newline parameters must not be provided.

  In text mode, a `ZstdFile` object is created, and wrapped in an `io.TextIOWrapper` instance with the specified encoding, error handling behavior, and line endings.

class compression.zstd.ZstdFile(file, /, mode='rb', *, level=None, options=None, zstd_dict=None)

  Open a Zstandard-compressed file in binary mode.

  A `ZstdFile` can wrap an already-open file object, or operate directly on a named file. The file argument specifies either the file object to wrap, or the name of the file to open (as a `str`, `bytes` or path-like object). If wrapping an existing file object, the wrapped file will not be closed when the `ZstdFile` is closed.

  The mode argument can be either `'rb'` for reading (default), `'wb'` for overwriting, `'xb'` for exclusive creation, or `'ab'` for appending. These can equivalently be given as `'r'`, `'w'`, `'x'` and `'a'` respectively.

  If file is a file object (rather than an actual file name), a mode of `'w'` does not truncate the file, and is instead equivalent to `'a'`.

  When reading, the options argument can be a dictionary providing advanced decompression parameters; see `DecompressionParameter` for detailed information about supported parameters. The zstd_dict argument is a `ZstdDict` instance to be used during decompression. When reading, if the level argument is not None, a `TypeError` will be raised.

  When writing, the options argument can be a dictionary providing advanced decompression parameters; see `CompressionParameter` for detailed information about supported parameters. The level argument is the compression level to use when writing compressed data. Only one of level or options may be passed. The zstd_dict argument is a `ZstdDict` instance to be used during compression.

  `ZstdFile` supports all the members specified by `io.BufferedIOBase`, except for `detach()` and `truncate()`. Iteration and the `with` statement are supported.

  The following method and attributes are also provided:

  peek(size=-1)

    Return buffered data without advancing the file position. At least one byte of data will be returned, unless EOF has been reached. The exact number of bytes returned is unspecified (the size argument is ignored).

    Note

    While calling `peek()` does not change the file position of the `ZstdFile`, it may change the position of the underlying file object (for example, if the `ZstdFile` was constructed by passing a file object for file).

  mode

    `'rb'` for reading and `'wb'` for writing.

  name

    The name of the Zstandard file. Equivalent to the `name` attribute of the underlying file object.

## Compressing and decompressing data in memory

compression.zstd.compress(data, level=None, options=None, zstd_dict=None)

  Compress data (a bytes-like object), returning the compressed data as a `bytes` object.

  The level argument is an integer controlling the level of compression. level is an alternative to setting `CompressionParameter.compression_level` in options. Use `bounds()` on `compression_level` to get the values that can be passed for level. If advanced compression options are needed, the level argument must be omitted and in the options dictionary the `CompressionParameter.compression_level` parameter should be set.

  The options argument is a Python dictionary containing advanced compression parameters. The valid keys and values for compression parameters are documented as part of the `CompressionParameter` documentation.

  The zstd_dict argument is an instance of `ZstdDict` containing trained data to improve compression efficiency. The function `train_dict()` can be used to generate a Zstandard dictionary.

compression.zstd.decompress(data, zstd_dict=None, options=None)

  Decompress data (a bytes-like object), returning the uncompressed data as a `bytes` object.

  The options argument is a Python dictionary containing advanced decompression parameters. The valid keys and values for compression parameters are documented as part of the `DecompressionParameter` documentation.

  The zstd_dict argument is an instance of `ZstdDict` containing trained data used during compression. This must be the same Zstandard dictionary used during compression.

  If data is the concatenation of multiple distinct compressed frames, decompress all of these frames, and return the concatenation of the results.

class compression.zstd.ZstdCompressor(level=None, options=None, zstd_dict=None)

  Create a compressor object, which can be used to compress data incrementally.

  For a more convenient way of compressing a single chunk of data, see the module-level function `compress()`.

  The level argument is an integer controlling the level of compression. level is an alternative to setting `CompressionParameter.compression_level` in options. Use `bounds()` on `compression_level` to get the values that can be passed for level. If advanced compression options are needed, the level argument must be omitted and in the options dictionary the `CompressionParameter.compression_level` parameter should be set.

  The options argument is a Python dictionary containing advanced compression parameters. The valid keys and values for compression parameters are documented as part of the `CompressionParameter` documentation.

  The zstd_dict argument is an optional instance of `ZstdDict` containing trained data to improve compression efficiency. The function `train_dict()` can be used to generate a Zstandard dictionary.

  compress(data, mode=ZstdCompressor.CONTINUE)

    Compress data (a bytes-like object), returning a `bytes` object with compressed data if possible, or otherwise an empty `bytes` object. Some of data may be buffered internally, for use in later calls to `compress()` and `flush()`. The returned data should be concatenated with the output of any previous calls to `compress()`.

    The mode argument is a `ZstdCompressor` attribute, either `CONTINUE`, `FLUSH_BLOCK`, or `FLUSH_FRAME`.

    When all data has been provided to the compressor, call the `flush()` method to finish the compression process. If `compress()` is called with mode set to `FLUSH_FRAME`, `flush()` should not be called, as it would write out a new empty frame.

  flush(mode=ZstdCompressor.FLUSH_FRAME)

    Finish the compression process, returning a `bytes` object containing any data stored in the compressor’s internal buffers.

    The mode argument is a `ZstdCompressor` attribute, either `FLUSH_BLOCK`, or `FLUSH_FRAME`.

  CONTINUE

    Collect more data for compression, which may or may not generate output immediately. This mode optimizes the compression ratio by maximizing the amount of data per block and frame.

  FLUSH_BLOCK

    Complete and write a block to the data stream. The data returned so far can be immediately decompressed. Past data can still be referenced in future blocks generated by calls to `compress()`, improving compression.

  FLUSH_FRAME

    Complete and write out a frame. Future data provided to `compress()` will be written into a new frame and cannot reference past data.

class compression.zstd.ZstdDecompressor(zstd_dict=None, options=None)

  Create a decompressor object, which can be used to decompress data incrementally.

  For a more convenient way of decompressing an entire compressed stream at once, see the module-level function `decompress()`.

  The options argument is a Python dictionary containing advanced decompression parameters. The valid keys and values for compression parameters are documented as part of the `DecompressionParameter` documentation.

  The zstd_dict argument is an instance of `ZstdDict` containing trained data used during compression. This must be the same Zstandard dictionary used during compression.

  Note

  This class does not transparently handle inputs containing multiple compressed frames, unlike the `decompress()` function and `ZstdFile` class. To decompress a multi-frame input, you should use `decompress()`, `ZstdFile` if working with a file object, or multiple `ZstdDecompressor` instances.

  decompress(data, max_length=-1)

    Decompress data (a bytes-like object), returning uncompressed data as bytes. Some of data may be buffered internally, for use in later calls to `decompress()`. The returned data should be concatenated with the output of any previous calls to `decompress()`.

    If max_length is non-negative, the method returns at most max_length bytes of decompressed data. If this limit is reached and further output can be produced, the `needs_input` attribute will be set to `False`. In this case, the next call to `decompress()` may provide data as `b''` to obtain more of the output.

    If all of the input data was decompressed and returned (either because this was less than max_length bytes, or because max_length was negative), the `needs_input` attribute will be set to `True`.

    Attempting to decompress data after the end of a frame will raise a `ZstdError`. Any data found after the end of the frame is ignored and saved in the `unused_data` attribute.

  eof

    `True` if the end-of-stream marker has been reached.

  unused_data

    Data found after the end of the compressed stream.

    Before the end of the stream is reached, this will be `b''`.

  needs_input

    `False` if the `decompress()` method can provide more decompressed data before requiring new compressed input.

## Zstandard dictionaries

compression.zstd.train_dict(samples, dict_size)

  Train a Zstandard dictionary, returning a `ZstdDict` instance. Zstandard dictionaries enable more efficient compression of smaller sizes of data, which is traditionally difficult to compress due to less repetition. If you are compressing multiple similar groups of data (such as similar files), Zstandard dictionaries can improve compression ratios and speed significantly.

  The samples argument (an iterable of `bytes` objects), is the population of samples used to train the Zstandard dictionary.

  The dict_size argument, an integer, is the maximum size (in bytes) the Zstandard dictionary should be. The Zstandard documentation suggests an absolute maximum of no more than 100 KB, but the maximum can often be smaller depending on the data. Larger dictionaries generally slow down compression, but improve compression ratios. Smaller dictionaries lead to faster compression, but reduce the compression ratio.

compression.zstd.finalize_dict(zstd_dict, /, samples, dict_size, level)

  An advanced function for converting a “raw content” Zstandard dictionary into a regular Zstandard dictionary. “Raw content” dictionaries are a sequence of bytes that do not need to follow the structure of a normal Zstandard dictionary.

  The zstd_dict argument is a `ZstdDict` instance with the `dict_content` containing the raw dictionary contents.

  The samples argument (an iterable of `bytes` objects), contains sample data for generating the Zstandard dictionary.

  The dict_size argument, an integer, is the maximum size (in bytes) the Zstandard dictionary should be. See `train_dict()` for suggestions on the maximum dictionary size.

  The level argument (an integer) is the compression level expected to be passed to the compressors using this dictionary. The dictionary information varies for each compression level, so tuning for the proper compression level can make compression more efficient.

class compression.zstd.ZstdDict(dict_content, /, *, is_raw=False)

  A wrapper around Zstandard dictionaries. Dictionaries can be used to improve the compression of many small chunks of data. Use `train_dict()` if you need to train a new dictionary from sample data.

  The dict_content argument (a bytes-like object), is the already trained dictionary information.

  The is_raw argument, a boolean, is an advanced parameter controlling the meaning of dict_content. `True` means dict_content is a “raw content” dictionary, without any format restrictions. `False` means dict_content is an ordinary Zstandard dictionary, created from Zstandard functions, for example, `train_dict()` or the external zstd CLI.

  When passing a `ZstdDict` to a function, the `as_digested_dict` and `as_undigested_dict` attributes can control how the dictionary is loaded by passing them as the `zstd_dict` argument, for example, `compress(data, zstd_dict=zd.as_digested_dict)`. Digesting a dictionary is a costly operation that occurs when loading a Zstandard dictionary. When making multiple calls to compression or decompression, passing a digested dictionary will reduce the overhead of loading the dictionary.

  > Difference for compression
  > | Digested dictionary | Undigested dictionary
  > --- | --- | ---
  > Advanced parameters of the compressor which may be overridden by the dictionary’s parameters | `window_log`, `hash_log`, `chain_log`, `search_log`, `min_match`, `target_length`, `strategy`, `enable_long_distance_matching`, `ldm_hash_log`, `ldm_min_match`, `ldm_bucket_size_log`, `ldm_hash_rate_log`, and some non-public parameters. | None
  > `ZstdDict` internally caches the dictionary | Yes. It’s faster when loading a digested dictionary again with the same compression level. | No. If you wish to load an undigested dictionary multiple times, consider reusing a compressor object.

  If passing a `ZstdDict` without any attribute, an undigested dictionary is passed by default when compressing and a digested dictionary is generated if necessary and passed by default when decompressing.

  > dict_content

  >   The content of the Zstandard dictionary, a `bytes` object. It’s the same as the dict_content argument in the `__init__` method. It can be used with other programs, such as the `zstd` CLI program.

  > dict_id

  >   Identifier of the Zstandard dictionary, a non-negative int value.

  >   Non-zero means the dictionary is ordinary, created by Zstandard functions and following the Zstandard format.

  >   `0` means a “raw content” dictionary, free of any format restriction, used for advanced users.

  >   Note

  >   The meaning of `0` for `ZstdDict.dict_id` is different from the `dictionary_id` attribute to the `get_frame_info()` function.

  > as_digested_dict

  >   Load as a digested dictionary.

  > as_undigested_dict

  >   Load as an undigested dictionary.

## Advanced parameter control

class compression.zstd.CompressionParameter

  An `IntEnum` containing the advanced compression parameter keys that can be used when compressing data.

  The `bounds()` method can be used on any attribute to get the valid values for that parameter.

  Parameters are optional; any omitted parameter will have it’s value selected automatically.

  Example getting the lower and upper bound of `compression_level`:

      lower, upper = CompressionParameter.compression_level.bounds()

  Example setting the `window_log` to the maximum size:

      _lower, upper = CompressionParameter.window_log.bounds()
      options = {CompressionParameter.window_log: upper}
      compress(b'venezuelan beaver cheese', options=options)

  bounds()

    Return the tuple of int bounds, `(lower, upper)`, of a compression parameter. This method should be called on the attribute you wish to retrieve the bounds of. For example, to get the valid values for `compression_level`, one may check the result of `CompressionParameter.compression_level.bounds()`.

    Both the lower and upper bounds are inclusive.

  compression_level

    A high-level means of setting other compression parameters that affect the speed and ratio of compressing data. Setting the level to zero uses `COMPRESSION_LEVEL_DEFAULT`.

  window_log

    Maximum allowed back-reference distance the compressor can use when compressing data, expressed as power of two, `1 << window_log` bytes. This parameter greatly influences the memory usage of compression. Higher values require more memory but gain better compression values.

    A value of zero causes the value to be selected automatically.

  hash_log

    Size of the initial probe table, as a power of two. The resulting memory usage is `1 << (hash_log+2)` bytes. Larger tables improve compression ratio of strategies <= `dfast`, and improve compression speed of strategies > `dfast`.

    A value of zero causes the value to be selected automatically.

  chain_log

    Size of the multi-probe search table, as a power of two. The resulting memory usage is `1 << (chain_log+2)` bytes. Larger tables result in better and slower compression. This parameter has no effect for the `fast` strategy. It’s still useful when using `dfast` strategy, in which case it defines a secondary probe table.

    A value of zero causes the value to be selected automatically.

  search_log

    Number of search attempts, as a power of two. More attempts result in better and slower compression. This parameter is useless for `fast` and `dfast` strategies.

    A value of zero causes the value to be selected automatically.

  min_match

    Minimum size of searched matches. Larger values increase compression and decompression speed, but decrease ratio. Note that Zstandard can still find matches of smaller size, it just tweaks its search algorithm to look for this size and larger. For all strategies < `btopt`, the effective minimum is `4`; for all strategies > `fast`, the effective maximum is `6`.

    A value of zero causes the value to be selected automatically.

  target_length

    The impact of this field depends on the selected `Strategy`.

    For strategies `btopt`, `btultra` and `btultra2`, the value is the length of a match considered “good enough” to stop searching. Larger values make compression ratios better, but compresses slower.

    For strategy `fast`, it is the distance between match sampling. Larger values make compression faster, but with a worse compression ratio.

    A value of zero causes the value to be selected automatically.

  strategy

    The higher the value of selected strategy, the more complex the compression technique used by zstd, resulting in higher compression ratios but slower compression.

    See also

    `Strategy`

  enable_long_distance_matching

    Long distance matching can be used to improve compression for large inputs by finding large matches at greater distances. It increases memory usage and window size.

    `True` or `1` enable long distance matching while `False` or `0` disable it.

    Enabling this parameter increases default `window_log` to 128 MiB except when expressly set to a different value. This setting is enabled by default if `window_log` >= 128 MiB and the compression strategy >= `btopt` (compression level 16+).

  ldm_hash_log

    Size of the table for long distance matching, as a power of two. Larger values increase memory usage and compression ratio, but decrease compression speed.

    A value of zero causes the value to be selected automatically.

  ldm_min_match

    Minimum match size for long distance matcher. Larger or too small values can often decrease the compression ratio.

    A value of zero causes the value to be selected automatically.

  ldm_bucket_size_log

    Log size of each bucket in the long distance matcher hash table for collision resolution. Larger values improve collision resolution but decrease compression speed.

    A value of zero causes the value to be selected automatically.

  ldm_hash_rate_log

    Frequency of inserting/looking up entries into the long distance matcher hash table. Larger values improve compression speed. Deviating far from the default value will likely result in a compression ratio decrease.

    A value of zero causes the value to be selected automatically.

  checksum_flag

    A four-byte checksum using XXHash64 of the uncompressed content is written at the end of each frame. Zstandard’s decompression code verifies the checksum. If there is a mismatch a `ZstdError` exception is raised.

    `True` or `1` enable checksum generation while `False` or `0` disable it.

  dict_id_flag

    When compressing with a `ZstdDict`, the dictionary’s ID is written into the frame header.

    `True` or `1` enable storing the dictionary ID while `False` or `0` disable it.

  nb_workers

    Select how many threads will be spawned to compress in parallel. When `nb_workers` > 0, enables multi-threaded compression, a value of `1` means “one-thread multi-threaded mode”. More workers improve speed, but also increase memory usage and slightly reduce compression ratio.

    A value of zero disables multi-threading.

  job_size

    Size of a compression job, in bytes. This value is enforced only when `nb_workers` >= 1. Each compression job is completed in parallel, so this value can indirectly impact the number of active threads.

    A value of zero causes the value to be selected automatically.

  overlap_log

    Sets how much data is reloaded from previous jobs (threads) for new jobs to be used by the look behind window during compression. This value is only used when `nb_workers` >= 1. Acceptable values vary from 0 to 9.

    > - 0 means dynamically set the overlap amount

    > - 1 means no overlap

    > - 9 means use a full window size from the previous job

    Each increment halves/doubles the overlap size. “8” means an overlap of `window_size/2`, “7” means an overlap of `window_size/4`, etc.

class compression.zstd.DecompressionParameter

  An `IntEnum` containing the advanced decompression parameter keys that can be used when decompressing data. Parameters are optional; any omitted parameter will have it’s value selected automatically.

  The `bounds()` method can be used on any attribute to get the valid values for that parameter.

  Example setting the `window_log_max` to the maximum size:

      data = compress(b'Some very long buffer of bytes...')

      _lower, upper = DecompressionParameter.window_log_max.bounds()

      options = {DecompressionParameter.window_log_max: upper}
      decompress(data, options=options)

  bounds()

    Return the tuple of int bounds, `(lower, upper)`, of a decompression parameter. This method should be called on the attribute you wish to retrieve the bounds of.

    Both the lower and upper bounds are inclusive.

  window_log_max

    The base-two logarithm of the maximum size of the window used during decompression. This can be useful to limit the amount of memory used when decompressing data. A larger maximum window size leads to faster decompression.

    A value of zero causes the value to be selected automatically.

class compression.zstd.Strategy

  An `IntEnum` containing strategies for compression. Higher-numbered strategies correspond to more complex and slower compression.

  Note

  The values of attributes of `Strategy` are not necessarily stable across zstd versions. Only the ordering of the attributes may be relied upon. The attributes are listed below in order.

  The following strategies are available:

  fast
  dfast
  greedy
  lazy
  lazy2
  btlazy2
  btopt
  btultra
  btultra2

## Miscellaneous

compression.zstd.get_frame_info(frame_buffer)

  Retrieve a `FrameInfo` object containing metadata about a Zstandard frame. Frames contain metadata related to the compressed data they hold.

class compression.zstd.FrameInfo

  Metadata related to a Zstandard frame.

  decompressed_size

    The size of the decompressed contents of the frame.

  dictionary_id

    An integer representing the Zstandard dictionary ID needed for decompressing the frame. `0` means the dictionary ID was not recorded in the frame header. This may mean that a Zstandard dictionary is not needed, or that the ID of a required dictionary was not recorded.

compression.zstd.COMPRESSION_LEVEL_DEFAULT

  The default compression level for Zstandard: `3`.

compression.zstd.zstd_version_info

  Version number of the runtime zstd library as a tuple of integers (major, minor, release).

## Examples

Reading in a compressed file:

    from compression import zstd

    with zstd.open("file.zst") as f:
        file_content = f.read()

Creating a compressed file:

    from compression import zstd

    data = b"Insert Data Here"
    with zstd.open("file.zst", "w") as f:
        f.write(data)

Compressing data in memory:

    from compression import zstd

    data_in = b"Insert Data Here"
    data_out = zstd.compress(data_in)

Incremental compression:

    from compression import zstd

    comp = zstd.ZstdCompressor()
    out1 = comp.compress(b"Some data\n")
    out2 = comp.compress(b"Another piece of data\n")
    out3 = comp.compress(b"Even more data\n")
    out4 = comp.flush()
    # Concatenate all the partial results:
    result = b"".join([out1, out2, out3, out4])

Writing compressed data to an already-open file:

    from compression import zstd

    with open("myfile", "wb") as f:
        f.write(b"This data will not be compressed\n")
        with zstd.open(f, "w") as zstf:
            zstf.write(b"This *will* be compressed\n")
        f.write(b"Not compressed\n")

Creating a compressed file using compression parameters:

    from compression import zstd

    options = {
       zstd.CompressionParameter.checksum_flag: 1
    }
    with zstd.open("file.zst", "w", options=options) as f:
        f.write(b"Mind if I squeeze in?")