Replace Optional[T] with T for default args, that expect T and not None

[sayandipdutta]

In quite a few places, function parameters with defaults are annotated with typing.Optional[T] (where T could be str, int, ...), I propose changing these to T where it is deemed to be the expected type of the variable for the rest of the block, and explicit None is not valid.

typing.Optional docs says the following about default value type annotations.

Note that this is not the same concept as an optional argument, which is one that has a default. An optional argument with a default does not require the Optional qualifier on its type annotation just because it is optional.

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This is not merely due to aesthetics, in my experience, Optional qualifiers can create some annoying type checking issues with mypy. For example:
test.py

from typing import Optional
def foo(arg: str) -> None:
    pass

def bar(arg: Optional[str] = "") -> None:
    foo(arg)
    return

Running mypy:

> mypy .\test.py
.\test.py:6: error: Argument 1 to "foo" has incompatible type "Optional[str]"; expected "str"
Found 1 error in 1 file (checked 1 source file)

Then you will need to do one of the following:

from typing import cast
def bar(arg: Optional[str] = "") -> None:
    foo(cast(str, arg))
    return

or

def bar(arg: Optional[str] = "") -> None:
    assert arg is not None
    foo(arg)
    return

or

def bar(arg: Optional[str] = "") -> None:
    if isinstance(arg, str):
        foo(arg)
    return

Although a single cast , assert or isinstance may not be a big deal, if you have to use it repeatedly, especially in loops, it looks unclean and reduces performance.

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Originally posted by @sayandipdutta in #75 (comment)

Related: #75 (comment)

[FyzHsn]

@sayandipdutta If you'd like to work on this or have started already, assign yourself to this issue.

[sayandipdutta]

@FyzHsn I will start working on it tomorrow. But I don't have permission to assign this to myself.