Python One Line With Statement

The with statement replaces former try...finally blocks in Python. It ensures that clean-up code is executed. For example, it closes open files before leaving the block. Consider this code example (assuming this code is stored in a file named 'code.py'):

with open('code.py') as code:
    print(code.read())

The output of this code would be the code itself (for nerds: a piece of code that generates itself is called a Quine):

''' OUTPUT
with open('code.py') as code:
    print(code.read())

'''

No matter what goes wrong inside the with block, Python will close the open file before moving on in the code. This way, you don’t need to enclose the code with a try...except statement.

Single Expression ‘With’ Statement in One Line

Python One Line With Statement

Problem: Can you write the with statement in a single line of code?

Solution: Yes, you can write the with statement in a single line of code if the loop body consists only of one statement:

with open('code.py') as code: print(code.read())

In general, you can write any indentation block (like if statements, with environments, or while loops) in a single line of code if the body consists of only one statement.

Exercise: The following interactive code throws an error if you run it. Fix the bug and run the correct code!

Multi Expression ‘With’ Statement in One Line

If the body consists of multiple statements, you can use a semicolon between the different statements:

with open('code.py') as code:
    print('The code:')
    print(code.read())

The previous code block becomes:

with open('code.py') as code: print('The code:'); print(code.read())

Note that in this particular instance, the semantics actually change because the code reads its own source file! But in all other cases, the semantics remain the same.

As soon as you have nested blocks like a for loop inside a with block, you cannot use this approach anymore because the code would become ambiguous. Believe it or not but the indentation serves a real purpose in Python! 😉

Nested Indentation Blocks in a One-Line ‘With’ Statement

If you know the Finxter tutorials, you also know that I seldomly conclude with such a statement “XYZ is impossible” because in most cases, it isn’t. If you’re in doubt whether you can compress an algorithm into a single line of code—don’t. You can compress all algorithms into a single line!

In most cases, you can avoid nested blocks by using list comprehension (rather than a for loop) or the ternary operator (rather than an if block).

Consider the following example with a for loop inside a with block:

with open('code.py') as code:
    for i in range(10):
        print(code.read())

Problem: One-Linerize a nested with block!

Wrong Solution: Write it into a single line:

Syntax Error With Statement Single Line

Correct Solution: Replace the inner for loop with a list comprehension statement!

with open('code.py') as code: [print(code.read()) for i in range(10)]

While this code runs and solves the problem, please note that the chosen example does not make a lot of sense. The file is read only once—even if you place it into a for loop. The reason is that the file reader is done reading the file after the first iteration. In subsequent iterations it only reads the remaining characters (there aren’t any) so the output is not 10x only 1x the file contents.

Where to Go From Here?

Enough theory, let’s get some practice!

To become successful in coding, you need to get out there and solve real problems for real people. That’s how you can become a six-figure earner easily. And that’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?

Practice projects is how you sharpen your saw in coding!

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