Variable defined with with-statement available outside of with-block?

Consider the following example:

with open('a.txt') as f:
    pass
# Is f supposed to be defined here?

I have read the language docs (2.7) for with-statement as well as PEP-343, but as far as I can tell they don’t say anything on this matter.

In CPython 2.6.5 f does seem to be defined outside of the with-block, but I’d rather not rely on an implementation detail that could change.

Answers:

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Method 1

Yes, the context manager will be available outside the with statement and that is not implementation or version dependent. with statements do not create a new execution scope.

Method 2

the with syntax:

with foo as bar:
    baz()

is approximately sugar for:

try:
    bar = foo.__enter__()
    baz()
finally:
    if foo.__exit__(*sys.exc_info()) and sys.exc_info():
        raise

This is often useful. For example

import threading
with threading.Lock() as myLock:
    frob()

with myLock:
    frob_some_more()

the context manager may be of use more than once.

Method 3

In case f is a file, it will be appear closed outside the with statement.

For example, this

f = 42
print f
with open('6432134.py') as f:
    print f
print f

would print:

42
<open file '6432134.py', mode 'r' at 0x10050fb70>
<closed file '6432134.py', mode 'r' at 0x10050fb70>

You can find the details in PEP-0343 under the section Specification: The ‘with’ Statement. Python scope rules (which might be irritating) apply to f as well.

Method 4

To answer Heikki’s question in the comments: yes, this scoping behavior is part of the python language specification and will work on any and all compliant Pythons (which includes PyPy, Jython, and IronPython).

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0