According to PEP-343, a with statement translates from: with EXPR as VAR: BLOCK to: mgr = (EXPR) exit = type(mgr).__exit__ # Not calling it yet value = type(mgr).__enter__(mgr) exc = True try: try: VAR = value # Only if “as VAR” is present BLOCK except: # The exceptional case is handled here exc = False … Read more
You bring up a criticism that has been raised before1. The cleanup in this case is non-deterministic, but it will happen with CPython when the generator gets garbage collected. Your mileage may vary for other python implementations… Here’s a quick example: from __future__ import print_function import contextlib @contextlib.contextmanager def manager(): “””Easiest way to get a … Read more
For a start, it helps prevent the problem you’ve introduced in your try … finally … example. The way you’ve structured it, if an exception is thrown while trying to open the file then you will never bind an open file to the name f, leading to either a NameError in the finally clause (if … Read more
Another option is the “backports.tempfile” package on pypi: https://pypi.python.org/pypi/backports.tempfile Quoting the project’s description: “This package provides backports of new features in Python’s tempfile module under the backports namespace.” Install with: pip install backports.tempfile Then use it in your script: from backports import tempfile with tempfile.TemporaryDirectory() as temp_dir: # modify files in this dir # here … Read more
The __enter__ method should return the context object. with … as … uses the return value of __enter__ to determine what object to give you. Since your __enter__ returns nothing, it implicitly returns None, so test is None. with Test(‘first’) as test: print(f’in {test.name}’) test = Test(‘second’) with test: print(f’in {test.name}’) So test is none. … Read more
The context manager can optionally return an object, to be assigned to the identifier named by as. And it is the object returned by the __enter__ method that is assigned by as, not necessarily the context manager itself. Using as <identifier> helps when you create a new object, like the open() call does, but not … Read more
Yes, you can get the effect by adding a little more code. #!/usr/bin/env python class Clippy_Runner( dict ): def __init__( self ): pass def __call__( self, **kwargs ): self.update( kwargs ) return self def __enter__( self ): return self def __exit__( self, exc_type, exc_val, exc_tb ): self.clear() clippy_runner = Clippy_Runner() print clippy_runner.get(‘verbose’) # Outputs None … Read more
A StringIO instance is an open file already. The open command, on the other hand, only takes filenames, to return an open file. A StringIO instance is not suitable as a filename. Also, you don’t need to close a StringIO instance, so there is no need to use it as a context manager either. While … Read more
As of Python 3.10, it is now possible to parenthesize the whole group of context managers, as you originally tried: with (Dummy() as a, Dummy() as b, # comment about c Dummy() as c): pass This is also technically possible in 3.9, but in a sort of semi-documented limbo. On one hand, it’s documented as … Read more
if you want to stay DRY, use an include. {% if foo %} {% with a as b %} {% include “snipet.html” %} {% endwith %} {% else %} {% with bar as b %} {% include “snipet.html” %} {% endwith %} {% endif %} or, even better would be to write a method on … Read more