fileinput inplace does not work with pathlib.Path

[zmwangx]

BPO	31281
Nosy	@ericvsmith, @zmwangx
PRs	bpo-31281: Fix pathlib.Path incompatibility in fileinput #3208

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = <Date 2017-09-04.18:15:28.145>
created_at = <Date 2017-08-26.01:50:52.589>
labels = ['3.7', 'type-bug', 'library']
title = 'fileinput inplace does not work with pathlib.Path'
updated_at = <Date 2017-09-04.18:15:28.143>
user = 'https://github.com/zmwangx'

bugs.python.org fields:

activity = <Date 2017-09-04.18:15:28.143>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2017-09-04.18:15:28.145>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2017-08-26.01:50:52.589>
creator = 'zmwangx'
dependencies = []
files = []
hgrepos = []
issue_num = 31281
keywords = []
message_count = 3.0
messages = ['300860', '301231', '301237']
nosy_count = 2.0
nosy_names = ['eric.smith', 'zmwangx']
pr_nums = ['3208']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31281'
versions = ['Python 3.6', 'Python 3.7']

[zmwangx]

Consider

    import fileinput
    import pathlib
    with fileinput.input(files=(pathlib.Path('in.txt'),), inplace=True) as fp:
        for line in fp:
            print(line, end='')

which results in

    Traceback (most recent call last):
      File "./pathlib-fileinput.py", line 6, in <module>
        for line in fp:
      File "/Users/zmwang/.pyenv/versions/3.6.1/lib/python3.6/fileinput.py", line 250, in __next__
        line = self._readline()
      File "/Users/zmwang/.pyenv/versions/3.6.1/lib/python3.6/fileinput.py", line 331, in _readline
        self._filename + (self._backup or ".bak"))
    TypeError: unsupported operand type(s) for +: 'PosixPath' and 'str'

A trivial fix is converting the specified filename to str when assigning to self._filename:

-        self._filename = self._files[0]
+        self._filename = str(self._files[0])

[ericvsmith]

New changeset 06de1ae by ericvsmith (Zhiming Wang) in branch 'master':
bpo-31281: Fix pathlib.Path incompatibility in fileinput (gh-3208)
06de1ae

[ericvsmith]

I did not backport this to 3.6, because it depends on other changes that themselves have not been backported.