blackd is a small HTTP server that exposes Black’s functionality over a simple protocol. The main benefit of using it is to avoid paying 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 do 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.


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 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 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-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.

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 kind of error while trying to format the input. The response body contains a textual representation of the error.