About pprint's stream arg

[FichteFoll]

According to the docs:

An output stream may be set using the stream keyword; the only method used on the stream object is the file protocol’s write() method.

Currently the stream arg is specified as TextIO, which seems overly restrictive to me. I had my variable declared as IO[str], which supports the .write() method just fine and would completely suffice, so I believe that stream should at least be stripped down to IO[str].

However, since the code actually ony uses stream.write() (I checked the source to verify), IO[str] still seems overly restrictive with its ~20 methods.

Should a new private typing type be created in the stub that only defines write()?

Related: #163

[gvanrossum]

I think it's reasonable to switch this to IO[str] (and if you send a PR to do that I'll happily review it). I don't think we should introduce new classes for duck typing like SupportsWrite; it's fragile (there aren't many interfaces that are documented to call just write()).

Eventually we should introduce protocols and then we might be able to deal with this better.

[FichteFoll]

python/typing#11 sounds almost exactly like what I imagined for this. Alright, let's wait for Protocol support before thinking about a SupportsWrite protocol. I'll submit a pr for IO[str] in the meantime.

[FichteFoll]

So, I did some testing and found out that the Python 2 implementation of pprint uses literal strings, i.e. str bytestrings, while in Python 3 literal strings are also str but Unicode strings.

In Py3, IO[str] is undoubtedly the correct type, but what would I put in Python 2 to specifically target bytestrings? bytes or str?

Note that the 2.7 stub currently has IO[Any] 😕 .

[gvanrossum]

You seem to be making up terminology ("literal" strings usually refers to writing a string literal in source code, not to the type itself). But I understand your point. In Python 2 bytes is just an alias for str, and given that in Python 3 this uses IO[str] I think that that's also the best type for Python 2.

[FichteFoll]

The usage of the term "literal string" was intended, example: https://github.com/python/cpython/blob/02f91871c1a976530cd2c566cbd6c85ad37f97a4/Lib/pprint.py#L140. Either way, str is the default string type in Python 2, so that should be natural.