Why f'{None}_some_string' not throw an error?

Question

I wonder why f'{None}_some_string' not throw an error?

Here is some example to reproduce the problem:

s1 = None # <class 'NoneType'>
s2 = 'real_string' # <class 'str'>

s1 + s2 # TypeError: unsupported operand type(s) for +: 'NoneType' and 'str


f'{s1}_bla'

'None_bla'

f'{s2}_bla'

'real_string_bla'

Is it possible to make expressions like f'{s}_some_string' throw an error when s is None?

No. F-strings automatically convert all the embedded expressions to strings. So it's analogous to `str(s1) + str(s2)` — Barmar, Mar 12 '21 at 16:12
Don't use an f-string if you don't want automatic formatting. — Barmar, Mar 12 '21 at 16:13
@Barmar Then the question is why `str(None)` not throw an error? — mrgloom, Mar 12 '21 at 16:15
Because the `NoneType` class provides an `__str__()` method that returns the string `'None'` — Barmar, Mar 12 '21 at 16:15
@mrgloom Because `None.__str__()` is implemented and returns the string `'None'` — DeepSpace, Mar 12 '21 at 16:15
Why do you want it to throw an error within format? If you want to raise an exception, check the value before creating the f-string. — RufusVS, Mar 12 '21 at 16:17

Florian Fasmeyer · Accepted Answer · 2021-03-12T16:26:48.483

2

By default f-string apply the str() (string) function.

f'{None}_some_string'

is equivalent to...

f'{str(None)}_some_string}'

edited Mar 12 '21 at 16:26

answered Mar 12 '21 at 16:20

Florian Fasmeyer

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Why f'{None}_some_string' not throw an error?

1 Answers1