fmark fmark - 19 days ago 6
Python Question

What is the python "with" statement designed for?

I came across the Python

with
statement for the first time today. I've been using Python lightly for several months and didn't even know of its existence! Given its somewhat obscure status, I thought it would be worth asking:


  1. What is the Python
    with
    statement
    designed to be used for?

  2. What do
    you use it for?

  3. Are there any
    gotchas I need to be aware of, or
    common anti-patterns associated with
    its use? Any cases where it is better use
    try..finally
    than
    with
    ?

  4. Why isn't it used more widely?

  5. Which standard library classes are compatible with it?


Answer
  1. I believe this has already been answered by other users before me, so I only add it for the sake of completeness: the with statement simplifies exception handling by encapsulating common preparation and cleanup tasks in so-called context managers. More details can be found in PEP 343. For instance, the open statement is a context manager in itself, which lets you open a file, keep it open as long as the execution is in the context of the with statement where you used it, and close it as soon as you leave the context, no matter whether you have left it because of an exception or during regular control flow. The with statement can thus be used in ways similar to the RAII pattern in C++: some resource is acquired by the with statement and released when you leave the with context.

  2. Some examples are: opening files using with open(filename) as fp:, acquiring locks using with lock: (where lock is an instance of threading.Lock). You can also construct your own context managers using the contextmanager decorator from contextlib. For instance, I often use this when I have to change the current directory temporarily and then return to where I was:

    from contextlib import contextmanager
    import os
    
    @contextmanager
    def working_directory(path):
        current_dir = os.getcwd()
        os.chdir(path)
        try:
            yield
        finally:
            os.chdir(current_dir)
    
    with working_directory("data/stuff"):
        # do something within data/stuff
    # here I am back again in the original working directory
    

    Here's another example that temporarily redirects sys.stdin, sys.stdout and sys.stderr to some other file handle and restores them later:

    from contextlib import contextmanager
    import sys
    
    @contextmanager
    def redirected(**kwds):
        stream_names = ["stdin", "stdout", "stderr"]
        old_streams = {}
        try:
            for sname in stream_names:
                stream = kwds.get(sname, None)
                if stream is not None and stream != getattr(sys, sname):
                    old_streams[sname] = getattr(sys, sname)
                    setattr(sys, sname, stream)
            yield
        finally:
            for sname, stream in old_streams.iteritems():
                setattr(sys, sname, stream)
    
    with redirected(stdout=open("/tmp/log.txt", "w")):
         # these print statements will go to /tmp/log.txt
         print "Test entry 1"
         print "Test entry 2"
    # back to the normal stdout
    print "Back to normal stdout again"
    

    And finally, another example that creates a temporary folder and cleans it up when leaving the context:

    from tempfile import mkdtemp
    from shutil import rmtree
    
    @contextmanager
    def temporary_dir(*args, **kwds):
        name = mkdtemp(*args, **kwds)
        try:
            yield name
        finally:
            shutil.rmtree(name)
    
    with temporary_dir() as dirname:
        # do whatever you want
    
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