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pythonwith-statementconceptual
Paranaboy
Paranaboy

Reputation: 11

Should I have most of my code in the with statement? Python

I've noticed that most tutorials showing how to use a with statement they keep most of their code in that statement. Is there a benefit to keeping it open (other than accessing the contents again) rather than pulling the contents into a list and closing it as soon as possible.

Example:

with open('file.txt') as f:
    list = f.readlines()
    # Code that does stuff with list
# end of with open

# vs

with open('file.txt') as f:
    list = f.readlines()
# end of with open
# Code that does stuff with list

Upvotes: 0

Views: 156

Answers (2)

Dave
Dave

Reputation: 996

The with statement is generally used to ensure resources are cleaned up as soon as possible. So as a general principal, keeping the with statement open for the shortest time possible is best to keep resource usage low.

In your example, the resource is a open file, which has quite a low overhead. But some context managers (e.g. a database connection) might represent much higher resource usage and benefit from early cleanup.

This is only a general principal though, and legitimate reasons exist to keep the with statement open longer. For example:

Resource reuse

If you needed to use the file later in your code, keeping it open would avoid having to re-open it later.

More efficient resource use

In your example, you're reading the whole file into memory as a list. If you only needed to process each line once, you might benefit from a longer with statement and an iterative approach.

In other words, rather than this, (which loads the whole file into memory):

with open('file.txt') as f:
    lines = f.readlines()
for line in lines:
    print(line)

Use this (which only loads one line at a time into memory):

with open('file.txt') as f:
    for line in f:
        print(line)

In the second example, the with statement is held open longer, but the memory usage should be lower.

Upvotes: 2

ubersmurf
ubersmurf

Reputation: 54

In Python, the with statement replaces a try-catch block with a concise shorthand. More importantly, it ensures closing resources right after processing them. So when you process your txt file(for this example) you don't need to keep it open after processing. Because your list is generated and saved. It's much more efficient for big files to prevent high memory usage. If the file is not external there is no need for with statement because it's already in this program.

What is internal and external format? Internal format: The internal format is the way the data is stored in the program. External format: The external format is the way the data is stored in files. For this example file.txt is external file and list is internal.

Upvotes: 2

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