SIM116: Use a dictionary instead many if/elif equality checks

[MartinThoma]

Explanation

• If you use a dictionary, you can easily see all expected parameters with `dict.keys()
• The formatting typically is slightly cleaner

Example

# Bad
if a == "foo":
    return "bar"
elif a == "bar":
    return "baz"
elif a == "boo":
    return "ooh"
else:
    return 42

# Good
mapping = {"foo": "bar", "bar": "baz", "boo": "ooh"}
return mapping.get(a, 42)

[MartinThoma]

        If(
            test=Compare(
                left=Name(id='a', ctx=Load()),
                ops=[Eq()],
                comparators=[Constant(value='foo', kind=None)],
            ),
            body=[
                Return(
                    value=Constant(value='bar', kind=None),
                ),
            ],
            orelse=[
                If(
                    test=Compare(
                        left=Name(id='a', ctx=Load()),
                        ops=[Eq()],
                        comparators=[Constant(value='bar', kind=None)],
                    ),
                    body=[
                        Return(
                            value=Constant(value='baz', kind=None),
                        ),
                    ],
                    orelse=[
                        If(
                            test=Compare(
                                left=Name(id='a', ctx=Load()),
                                ops=[Eq()],
                                comparators=[Constant(value='boo', kind=None)],
                            ),
                            body=[
                                Return(
                                    value=Constant(value='ooh', kind=None),
                                ),
                            ],
                            orelse=[
                                Return(
                                    value=Constant(value=42, kind=None),
                                ),
                            ],
                        ),
                    ],
                ),
            ],

[Effervex]

This style guideline may result in better looking code, but it is inefficient and can even create errors. Each dict value is interpreted whenever the dict is evaluated.

For example:

some_array = [0,1] if x else [0,1,2]
mapping = {
  "foo": "bar",
  "foo2": long_expensive_function()
  "foo3": three_arg_func(some_array[0], some_array[1], some_array[2])
}
return mapping.get("foo",42) # This will still evaluate "foo2", and if x is True, completely crash when evaluating "foo3"

Edit: I created a new issue and referenced this.