Black as a server (blackd)#

blackd is a small HTTP server that exposes Black’s functionality over a simple protocol. The main benefit of using it is to avoid the cost of starting up a new Black process every time you want to blacken a file.


blackd is not packaged alongside Black by default because it has additional dependencies. You will need to execute pip install 'black[d]' to install it.

You can start the server on the default port, binding only to the local interface by running blackd. You will see a single line mentioning the server’s version, and the host and port it’s listening on. blackd will then print an access log similar to most web servers on standard output, merged with any exception traces caused by invalid formatting requests.

blackd provides even less options than Black. You can see them by running blackd --help:

Usage: blackd [OPTIONS]

  --bind-host TEXT     Address to bind the server to.
  --bind-port INTEGER  Port to listen on
  --version            Show the version and exit.
  -h, --help           Show this message and exit.

There is no official blackd client tool (yet!). You can test that blackd is working using curl:

blackd --bind-port 9090 &  # or let blackd choose a port
curl -s -XPOST "localhost:9090" -d "print('valid')"


blackd only accepts POST requests at the / path. The body of the request should contain the python source code to be formatted, encoded according to the charset field in the Content-Type request header. If no charset is specified, blackd assumes UTF-8.

There are a few HTTP headers that control how the source code is formatted. These correspond to command line flags for Black. There is one exception to this: X-Protocol-Version which if present, should have the value 1, otherwise the request is rejected with HTTP 501 (Not Implemented).

The headers controlling how source code is formatted are:

  • X-Line-Length: corresponds to the --line-length command line flag.

  • X-Skip-String-Normalization: corresponds to the --skip-string-normalization command line flag. If present and its value is not the empty string, no string normalization will be performed.

  • X-Skip-Magic-Trailing-Comma: corresponds to the --skip-magic-trailing-comma command line flag. If present and its value is not the empty string, trailing commas will not be used as a reason to split lines.

  • X-Fast-Or-Safe: if set to fast, blackd will act as Black does when passed the --fast command line flag.

  • X-Python-Variant: if set to pyi, blackd will act as Black does when passed the --pyi command line flag. Otherwise, its value must correspond to a Python version or a set of comma-separated Python versions, optionally prefixed with py. For example, to request code that is compatible with Python 3.5 and 3.6, set the header to py3.5,py3.6.

  • X-Diff: corresponds to the --diff command line flag. If present, a diff of the formats will be output.

If any of these headers are set to invalid values, blackd returns a HTTP 400 error response, mentioning the name of the problematic header in the message body.

Apart from the above, blackd can produce the following response codes:

  • HTTP 204: If the input is already well-formatted. The response body is empty.

  • HTTP 200: If formatting was needed on the input. The response body contains the blackened Python code, and the Content-Type header is set accordingly.

  • HTTP 400: If the input contains a syntax error. Details of the error are returned in the response body.

  • HTTP 500: If there was any other kind of error while trying to format the input. The response body contains a textual representation of the error.

The response headers include a X-Black-Version header containing the version of Black.