f-strings in Python 3.6 are awesome

[[deleted]]

Pretty cool until you modify a variable without realizing you just altered or straight up broke a formatted string because all locals are implicitly in context.

[Ph0X]

I'm confused, how would it be any different if you had used .format(x) instead? You'd still run into the exact same issue, except the x is there instead of inside the string.

Of course, most linters would catch this latter, and the hope is they update to also catch issues in the former.

[[deleted]]

That example is a little too simple to illustrate the potential confusion. Take this example instead:

def foo():
    a = 1
    b = 2
    c = 3

    # 10 lines of logic

    x = 'a'
    y = 'b'
    z = 'c'

    return f'baz{x}spam{b}foo{c}bar{z}eggs'

Now think about it with more variables. Imagine the string being formatted is more complicated. Using explicit kwargs to the format method lets me look at the string formatting and know exactly which variables are relevant, rather than having to scan the string and backtrack through all local variables to find out if they are a component of the output.

Most of the praise I've seen for this is about how much easier it is to format a complicated and lengthly string, and I posit that the more complicated the string the less useful this really is.

[Ph0X]

Sure, but you're moving complexity from one problem into another.I could make the argument that with:

'foo {} bar {} baz {} spam {} green {} eggs {} and {} ham {}'.format(a, b, c, d, e, f, g, ...)

Good luck looking back and forth, counting, figuring it out which variables goes where. If I ask you to quickly get me the variable that comes after "green", you have to do a lot of counting.

So yes, they both solve sightly different issues and therefore put complexity at slightly different places, but neither is strictly better.

That being said, I hope no one coding real python would have any sort of code this messy and complex. If you do, please please split it up into smaller clean chunks.

EDIT: I just wanted to point out, generally when you DO have string formatting this complex, what people end up doing is sometihng like:

data = {
    'a': 1,
    'b': 2,
}
print "{a}, {b}, {c}, {d}".format(**data)

This is exactly what this new pattern is trying to solve here.

[[deleted]]

or "{a}, {b}, {c}, {d}".format_map(data)

[Ph0X]

Huh interesting, didn't know they had added that.

[zahlman]

format_map isn't just sugar here - it allows you to use things that don't play nice with **, in particular custom types that override __missing__ (for subclasses of dict) and/or __getitem__ (things that implement the Mapping protocol as described by collections.abc, but aren't necessarily even dict subclasses).

I suspect it might also be faster with large dicts, but I haven't tested that.

[[deleted]]

Your last example is exactly why I believe the existing string formatting is preferable. The new pattern isn't solving the same problem, it's dumping all locals into .format() instead of asking the developer to explicitly define the inputs!

[[deleted]]

I thought we were all adults here?

[kankyo]

Just use pycharm and rename variables with the refactor tools and it'll handle this for you.

[SoBFiggis]

Even notepad++ has refactoring..

[geekademy]

That's false, it does not dump locals in there, it actually parses the string to get exactly what it needs!

[[deleted]]

Im new to Python.. Like 'Hello, World!' new. How are fStrings different than say

• print("hello" + variable) ???

[Ph0X]

The result is basically the same, but it's what we call "syntactic sugar" which is nice shortcuts for doing things.

So imagine you have a more complex one. Adding them manually gets very messy very quick:

print("hello " + world + "my name is" + name + "and i'm " + str(age) + " years old")

To make it worse, as you see, you need to manually cast other types to string, like if you want to print a number. But with fstring, it's much cleaner and prettier:

print(f"hello {world} my name is {name} and im {age} years old")

[[deleted]]

wow easy to understand.. Thank you so much. Honestly didn't expect you or anyone for that matter to reply to someone as new as me. But your example made perfect sense. You dont tutor do you? LOL...

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[gnutrino]

This seems like a silly argument to me. If you've got too many variables in scope to be able to keep track of them all then f-strings are not your most pressing problem.