Windows Console Notes

New in version 6.0.

Until Click 6.0 there are various bugs and limitations with using Click on a Windows console. Most notably the decoding of command line arguments was performed with the wrong encoding on Python 2 and on all versions of Python output of unicode characters was impossible. Starting with Click 6.0 we now emulate output streams on Windows to support unicode to the Windows console through separate APIs and we perform different decoding of parameters.

Here is a brief overview of how this works and what it means to you.

Unicode Arguments

Click internally is generally based on the concept that any argument can come in as either byte string or unicode string and conversion is performed to the type expected value as late as possible. This has some advantages as it allows us to accept the data in the most appropriate form for the operating system and Python version.

For instance paths are left as bytes on Python 2 unless you explicitly tell it otherwise.

This caused some problems on Windows where initially the wrong encoding was used and garbage ended up in your input data. We not only fixed the encoding part, but we also now extract unicode parameters from sys.argv.

This means that on Python 2 under Windows, the arguments processed will most likely be of unicode nature and not bytes. This was something that previously did not really happen unless you explicitly passed in unicode parameters so your custom types need to be aware of this.

There is also another limitation with this: if sys.argv was modified prior to invoking a click handler, we have to fall back to the regular byte input in which case not all unicode values are available but only a subset of the codepage used for parameters.

Unicode Output and Input

Unicode output and input on Windows is implemented through the concept of a dispatching text stream. What this means is that when click first needs a text output (or input) stream on windows it goes through a few checks to figure out of a windows console is connected or not. If no Windows console is present then the text output stream is returned as such and the encoding for that stream is set to utf-8 like on all platforms.

However if a console is connected the stream will instead be emulated and use the cmd.exe unicode APIs to output text information. In this case the stream will also use utf-16-le as internal encoding. However there is some hackery going on that the underlying raw IO buffer is still bypassing the unicode APIs and byte output through an indirection is still possible.

This hackery is used on both Python 2 and Python 3 as neither version of Python has native support for cmd.exe with unicode characters. There are some limitations you need to be aware of:

• This unicode support is limited to click.echo, click.prompt as well as click.get_text_stream.

• Depending on if unicode values or byte strings are passed the control flow goes completely different places internally which can have some odd artifacts if data partially ends up being buffered. Click attempts to protect against that by manually always flushing but if you are mixing and matching different string types to stdout or stderr you will need to manually flush.

• The raw output stream is set to binary mode, which is a global operation on Windows, so print calls will be affected. Prefer click.echo over print.

• On Windows 7 and below, there is a limitation where at most 64k characters can be written in one call in binary mode. In this situation, sys.stdout and sys.stderr are replaced with wrappers that work around the limitation.

Another important thing to note is that the Windows console’s default fonts do not support a lot of characters which means that you are mostly limited to international letters but no emojis or special characters.