Use fspath to fully support PathLike objects.

PathLike objects were already implicitly supported by `input` and by
`output` when the filename was explicitly specified as a keyword
argument. However, implicitly setting the output filename from a list of
output streams/filename did not permit a PathLike object.

ffmpeg/_ffmpeg.py

@@ -1,7 +1,7 @@
 from __future__ import unicode_literals
 
 from past.builtins import basestring
-from ._utils import basestring
+from ._utils import basestring, fspath
 
 from .nodes import (
     filter_operator,
@@ -23,7 +23,7 @@ def input(filename, **kwargs):
 
     Official documentation: `Main options <https://ffmpeg.org/ffmpeg.html#Main-options>`__
     """
-    kwargs['filename'] = filename
+    kwargs['filename'] = fspath(filename)
     fmt = kwargs.pop('f', None)
     if fmt:
         if 'format' in kwargs:
@@ -79,9 +79,12 @@ def output(*streams_and_filename, **kwargs):
     """
     streams_and_filename = list(streams_and_filename)
     if 'filename' not in kwargs:
-        if not isinstance(streams_and_filename[-1], basestring):
+        # Raise any errors without destructively modifying streams_and_filenames
+        try:
+            fspath(streams_and_filename[-1])
+        except TypeError:
             raise ValueError('A filename must be provided')
-        kwargs['filename'] = streams_and_filename.pop(-1)
+        kwargs['filename'] = fspath(streams_and_filename.pop(-1))
     streams = streams_and_filename
 
     fmt = kwargs.pop('f', None)

ffmpeg/_utils.py

@@ -106,3 +106,38 @@ def convert_kwargs_to_cmd_line_args(kwargs):
         if v is not None:
             args.append('{}'.format(v))
     return args
+
+
+if sys.version_info >= (3, 6):
+    from os import fspath
+else:
+    # This code is mostly copy-pasted from PEP 519, with (str, bytes) instance
+    # checks converted to basestring instance checks.
+    def fspath(path):
+        """Return the string representation of the path.
+
+        If str or bytes is passed in, it is returned unchanged. If __fspath__()
+        returns something other than str or bytes then TypeError is raised. If
+        this function is given something that is not str, bytes, or os.PathLike
+        then TypeError is raised.
+        """
+        if isinstance(path, basestring):
+            return path
+
+        # Work from the object's type to match method resolution of other magic
+        # methods.
+        path_type = type(path)
+        try:
+            path = path_type.__fspath__(path)
+        except AttributeError:
+            if hasattr(path_type, '__fspath__'):
+                raise
+        else:
+            if isinstance(path, basestring):
+                return path
+            else:
+                raise TypeError("expected __fspath__() to return str or bytes, "
+                                "not " + type(path).__name__)
+
+        raise TypeError("expected str, bytes or os.PathLike object, not "
+                        + path_type.__name__)