Is it safe to combine 'with' and 'yield' in python?

Question

It\'s a common idiom in python to use context manager to automatically close files:

with open(\'filename\') as my_file:
    # do something with my_file

# my_fil         


        

Answer 1

Is it safe to combine 'with' and 'yield' in python?

I don't think you should do this.

Let me demonstrate making some files:

>>> for f in 'abc':
...     with open(f, 'w') as _: pass

Convince ourselves that the files are there:

>>> for f in 'abc': 
...     with open(f) as _: pass 

And here's a function that recreates your code:

def gen_abc():
    for f in 'abc':
        with open(f) as file:
            yield file

Here it looks like you can use the function:

>>> [f.closed for f in gen_abc()]
[False, False, False]

But let's create a list comprehension of all of the file objects first:

>>> l = [f for f in gen_abc()]
>>> l
[<_io.TextIOWrapper name='a' mode='r' encoding='cp1252'>, <_io.TextIOWrapper name='b' mode='r' encoding='cp1252'>, <_io.TextIOWrapper name='c' mode='r' encoding='cp1252'>]

And now we see they are all closed:

>>> c = [f.closed for f in l]
>>> c
[True, True, True]

This only works until the generator closes. Then the files are all closed.

I doubt that is what you want, even if you're using lazy evaluation, your last file will probably be closed before you're done using it.