Add a "Patterns" chapter

[matrixise]

Yesterday, I have fixed an issue in bpo and this issue was for the support of os.PathLike in FileCookieJar. Since the introduction of os.PathLike and __fspath__, we continue to convert some parts of code to be os.PathLike compliant and I think there is a pattern for that.

Maybe we could create a page "Patterns" with some examples.

For example (but need to be tested and approved)

How to support a os.PathLike object?

def __init__(self, filename):
    try:
        filename = os.fspath(filename)
    except TypeError:
        raise ValueError("filename must be a string-like or path-like object")

[Mariatta]

GitMate.io thinks possibly related issues are python/core-workflow#189 (Add CONTRIBUTING.md), #3 (Add Travis support), #172 (Add PDF build), #136 (Add a chapter for introducing common function / class' corresponding source file), and python/core-workflow#32 (Add the mention bot?).

[ncoghlan]

Why are we converting an accurate TypeError to an incorrect ValueError? (If the passed in object doesn't adhere to what os.fspath supports, changing the value passed in won't help - you have to pass in an argument of a different type)

[matrixise]

@ncoghlan good catch

my example comes from cookiejar where there was a check and if there was an exception this one was catched and re-raised as a ValueError

I could fix it in cookiejar but I don't want to break the backward-compatiblity, or maybe I could document it in the whatsnew/38.html

I am going to fix my PR with your recommendation in CPython: https://github.com/python/cpython/pull/11945/files#diff-e85eca61cec5281c45f14cd6819bf548L1766

Thank you

[matrixise]

Ok I have fixed my PR python/cpython#11945

Thank you

[brettcannon]

I think this is out of scope for the devguide. Patterns like this are a general thing for the wider community and should be covered elsewhere, e.g. blog posts or the documentation.