Nick Coghlan <[email protected]> added the comment:
TestCase.setUp() and TestCase.tearDown() were amongst the precursors
to__enter__() and __exit__(). addCleanUp() fills exactly the same role here -
and I've seen *plenty* of positive feedback directed towards Michael for that
addition to the unittest API.
For individual one-off cases, a flag variable and an if statement inside a
finally block is an adequate, but not ideal, solution, because it suffers from
all the readability and auditability problems of *any* try/finally based
solution. It's particularly annoying when an object *does* support the context
management protocol, but I can't use a with statement simply because I don't
*always* need (and/or own) that resource (this kind of thing happens in a few
places in runpy, since the behaviour changes depending on whether or not runpy
created temporary objects for itself or was given objects as arguments)
Custom context managers are typically a bad idea in these circumstances,
because they make readability *worse* (relying on people to understand what the
context manager does). A standard library based solution, on the other hand,
offers the best of both worlds:
- code becomes easier to write correctly and to audit for correctness (for all
the reasons with statements were added in the first place)
- the idiom will eventually become familiar to all Python users
If other "cleanup function" registration APIs didn't already exist, I'd agree
with you that this needed further exposure. However, I simply don't agree
that's the case - atexit and addCleanup provide your field testing, the rest of
the design is just a matter of integrating those concepts with the context
management protocol.
Indeed, one of the objections I received after we deprecated
contextlib.nested() was that you couldn't easily pass a programmatically
generated list of resources to nested with statements. Given
contextlib.CleanupManager it becomes trivial:
with contextlib.CleanupManager as cm:
files = [cm.enter_context(open(fname)) for fname in names]
# All files will be closed when we leave the context
I can take this up on python-dev if you want, but I hope to persuade you that
the desire *is* there, it's just that the workarounds for the lack of this
functionality involve avoiding the context management protocol entirely:
try:
files = [open(fname) for fname in names]
# Are all files closed when we're done?
# I dunno, scroll down past the algorithm code to check!
# Avoiding this would be good for all the reasons the
# with statement was added in the first place
finally:
for f in files:
f.close()
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