Nick Coghlan added the comment:
As Raymond noted, we should resist the temptation to generalise this too much -
generalisation almost always comes at the cost of making any *specific* case
harder (consider the difference between the complexity of the "support any URL
scheme" model in urllib and urllib2 and the relatively API simplicity of the
higher level HTTP/HTTPS focused requests module).
I'm wary of calling it "PrintRedirect" though, as I believe that's actively
misleading: swapping sys.stdout for something else affects more than just the
print builtin, and if a particular print call uses the "file" parameter, then
the redirection of sys.stdout will have no effect.
"Redirection" in general is a bit of a misnomer for what the context manager is
doing.
So, here's a concrete API suggestion and implementation:
def replace_stdin(stream=None, *, data=None):
if data is not None:
if stream is not None:
raise ValueError("Cannot specify both stream & data")
stream = StringIO(data)
return ReplaceStandardStream('stdin', stream)
def replace_stdout(stream=None):
return ReplaceStandardStream('stdout', stream)
def replace_stderr(stream=None):
return ReplaceStandardStream('stderr', stream)
class ReplaceStandardStream:
"""Context manager to temporarily replace a standard stream
On entry, replaces the specified sys module stream attribute
('stdin', 'stdout' or 'stderr') with the supplied IO stream
object.
On exit, restores the previous value of the sys module
attribute.
Note: as this context manager modifies sys module attributes
directly, it is NOT thread-safe.
"""
def __init__(self, attr, stream):
if attr not in ('stdin', 'stdout', 'stderr'):
raise ValueError("{.200!r} is not a standard stream name
(expected one of: 'stdin', 'stdout', or 'stderr'".format(attr))
self._attr_to_replace = attr
self._old_stream = None
if stream is None:
self._replacement_stream = StringIO()
else:
self._replacement_stream = stream
def __enter__(self):
if self._old_stream is not None:
raise RuntimeError("Cannot reenter {!r}".format(self))
self._old_stream = getattr(sys, self._attr_to_replace)
stream = self._replacement_stream
setattr(sys, self._attr_to_replace, stream)
return stream
def __exit__(self):
stream = self._old_stream
if stream is None:
raise RuntimeError("Never entered {!r}".format(self))
self._old_stream = None
setattr(sys, self._attr_to_replace, stream)
Cross linking from the print documentation to io.replace_stdout and from the
input documentation to io.replace_stdin may also be a good idea.
----------
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue15805>
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