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Last if statement guard

Sourcery refactoring id: last-if-guard

Description:

Convert the final conditional into a guard clause

Before:

def f(a=None):
    if a is None:
        return 42
    else:
        # some long calculations
        var = (i 2 for i in range(a))
        return sum(var)

After:

def f(a=None):
    if a is None:
        return 42

    # some long calculations
    var = (i 2 for i in range(a))
    return sum(var)

Explanation:

A common code pattern is to have some clauses at the start of a function, to check whether certain conditions have been fulfilled and return early or raise an exception if not.

While this is perfectly valid code, it can run into problems with excessive nesting, particularly if the rest of the function is fairly long.

Here we can take advantage of the fact that we don't need the else if the main body of the if breaks the control flow by ending with return or raise. Rewriting the function as shown here is logically equivalent.

Using a guard condition, or multiple guard conditions, in this way now doesn't cause the rest of the function to be indented. In general the less we have to deal with indents the easier the code is to understand.

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