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comp / comp.lang.python / Re: Best use of "open" context manager

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o Re: Best use of "open" context managerThomas Passin

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Subject: Re: Best use of "open" context manager
From: Thomas Passin
Newsgroups: comp.lang.python
Date: Sat, 6 Jul 2024 13:40 UTC
References: 1 2
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From: list1@tompassin.net (Thomas Passin)
Newsgroups: comp.lang.python
Subject: Re: Best use of "open" context manager
Date: Sat, 6 Jul 2024 09:40:49 -0400
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On 7/6/2024 6:49 AM, Rob Cliffe via Python-list wrote:
> Consider this scenario (which I ran into in real life):
>     I want to open a text file and do a lot of processing on the lines
> of that file.
>     If the file does not exist I want to take appropriate action, e.g.
> print an error message and abort the program.
> I might write it like this:
>
> try:
>     with open(FileName) as f:
>         for ln in f:
>             print("I do a lot of processing here")
>             # Many lines of code here .....
> except FileNotFoundError:
>     print(f"File {FileName} not found")
>     sys.exit()
>
> but this violates the principle that a "try" suite should be kept small,
> so that only targeted exceptions are trapped,
> not to mention that having "try" and "except" far apart decreases
> readability.
>
> Or I might write it like this:
>
> try:
>     f = open(FileName) as f:
>     FileLines = f.readlines()
> except FileNotFoundError:
>     print(f"File {FileName} not found")
>     sys.exit()
> # I forgot to put "f.close()" here -:)
> for ln in File Lines:
>         print("I do a lot of processing here")
>         # Many lines of code here .....
>
> but this loses the benefits of using "open" as a context manager,
> and would also be unacceptable if the file was too large to read into
> memory.
>
> Really I would like to write something like
>
> try:
>     with open(FileName) as f:
> except FileNotFoundError:
>     print(f"File {FileName} not found")
>     sys.exit()
> else: # or "finally:"
>         for ln in f:
>             print("I do a lot of processing here")
>             # Many lines of code here .....
>
> but this of course does not work because by the time we get to "for ln
> in f:" the file has been closed so we get
> ValueError: I/O operation on closed file
>
> I could modify the last attempt to open the file twice, which would
> work, but seems like a kludge (subject to race condition, inefficient).
>
> Is there a better / more Pythonic solution?

I usually read the file into a sequence of lines and then leave the
open() as soon as possible. Something like this:

FILENAME = 'this_is_an_example.txt'
lines = None
if os.path.exists(FILENAME):
with open(FILENAME) as f:
lines = f.readlines()
# do something with lines

Of course, if you want to read a huge number of lines you will need to
be more thoughtful about it. Or make all the processing within the
open() block be a function. Then you just have one more line in the block.

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