Python 3 Support

Click supports Python 3, but like all other command line utility libraries, it suffers from the Unicode text model in Python 3. All examples in the documentation were written so that they could run on both Python 2.x and Python 3.3 or higher.

At the moment, it is strongly recommended is to use Python 2 for Click utilities unless Python 3 is a hard requirement.

Python 3 Limitations

At the moment, Click suffers from a few problems with Python 3:

  • The command line in Unix traditionally is in bytes, not Unicode. While there are encoding hints for all of this, there are generally some situations where this can break. The most common one is SSH connections to machines with different locales.

    Misconfigured environments can currently cause a wide range of Unicode problems in Python 3 due to the lack of support for roundtripping surrogate escapes. This will not be fixed in Click itself!

    For more information see Python 3 Surrogate Handling.

  • Standard input and output in Python 3 is opened in Unicode mode by default. Click has to reopen the stream in binary mode in certain situations. Because there is no standardized way to do this, this might not always work. Primarily this can become a problem when testing command-line applications.

    This is not supported:

    sys.stdin = io.StringIO('Input here')
    sys.stdout = io.StringIO()

    Instead you need to do this:

    input = 'Input here'
    in_stream = io.BytesIO(input.encode('utf-8'))
    sys.stdin = io.TextIOWrapper(in_stream, encoding='utf-8')
    out_stream = io.BytesIO()
    sys.stdout = io.TextIOWrapper(out_stream, encoding='utf-8')

    Remember that in that case, you need to use out_stream.getvalue() and not sys.stdout.getvalue() if you want to access the buffer contents as the wrapper will not forward that method.

Python 2 and 3 Differences

Click attempts to minimize the differences between Python 2 and Python 3 by following best practices for both languages.

In Python 2, the following is true:

  • sys.stdin, sys.stdout, and sys.stderr are opened in binary mode, but under some circumstances they support Unicode output. Click attempts to not subvert this but provides support for forcing streams to be Unicode-based.

  • sys.argv is always byte-based. Click will pass bytes to all input types and convert as necessary. The STRING type automatically will decode properly the input value into a string by trying the most appropriate encodings.

  • When dealing with files, Click will never go through the Unicode APIs and will instead use the operating system’s byte APIs to open the files.

In Python 3, the following is true:

  • sys.stdin, sys.stdout and sys.stderr are by default text-based. When Click needs a binary stream, it attempts to discover the underlying binary stream. See Python 3 Limitations for how this works.

  • sys.argv is always Unicode-based. This also means that the native type for input values to the types in Click is Unicode, and not bytes.

    This causes problems if the terminal is incorrectly set and Python does not figure out the encoding. In that case, the Unicode string will contain error bytes encoded as surrogate escapes.

  • When dealing with files, Click will always use the Unicode file system API calls by using the operating system’s reported or guessed filesystem encoding. Surrogates are supported for filenames, so it should be possible to open files through the File type even if the environment is misconfigured.

Python 3 Surrogate Handling

Click in Python 3 does all the Unicode handling in the standard library and is subject to its behavior. In Python 2, Click does all the Unicode handling itself, which means there are differences in error behavior.

The most glaring difference is that in Python 2, Unicode will “just work”, while in Python 3, it requires extra care. The reason for this is that in Python 3, the encoding detection is done in the interpreter, and on Linux and certain other operating systems, its encoding handling is problematic.

The biggest source of frustration is that Click scripts invoked by init systems (sysvinit, upstart, systemd, etc.), deployment tools (salt, puppet), or cron jobs (cron) will refuse to work unless a Unicode locale is exported.

If Click encounters such an environment it will prevent further execution to force you to set a locale. This is done because Click cannot know about the state of the system once it’s invoked and restore the values before Python’s Unicode handling kicked in.

If you see something like this error in Python 3:

Traceback (most recent call last):
RuntimeError: Click will abort further execution because Python 3 was
  configured to use ASCII as encoding for the environment. Either switch
  to Python 2 or consult for
  mitigation steps.

You are dealing with an environment where Python 3 thinks you are restricted to ASCII data. The solution to these problems is different depending on which locale your computer is running in.

For instance, if you have a German Linux machine, you can fix the problem by exporting the locale to de_DE.utf-8:

export LC_ALL=de_DE.utf-8
export LANG=de_DE.utf-8

If you are on a US machine, en_US.utf-8 is the encoding of choice. On some newer Linux systems, you could also try C.UTF-8 as the locale:

export LC_ALL=C.UTF-8
export LANG=C.UTF-8

You need to do this before you invoke your Python script. If you are curious about the reasons for this, you can join the discussions in the Python 3 bug tracker: