You could do something like `value = table.get(some_function(), MISSING_VALUE)` and then have the conditional. But let's say for the sake of argument, yeah you need to assign the value up-front.

Let's say you're looking at some code like this:

    if value in table:
        ...

If you need to change this so that it's `some_function(value)`, you're not going to miss the fact that you have a decision to make here: you can either assign a new variable, or you can call the function twice, or you can use the '.get()' approach.

If you instead have:

    try:
        return table[value]
    except KeyError:
        ...

You now have to consciously avoid writing the incorrect code `try: return table[some_function(value)]`. It's very easy to change the code in the 'try' block so that you introduce an unintended way to end up in the 'except' block.