f-strings formatter including for-loop or if conditions

问题

How can I insert for loops or if expressions inside an f-string?

I thought initially of doing something like this for if expressions:

f'{a:{"s" if CONDITION else "??"}}'

What I would like to do though is something like:

Example 1

f'{key: value\n for key, value in dict.items()}'

result:

if dict = {'a': 1, 'b': 2}

a: 1 
b: 2

or Example 2

c = 'hello'
f'{c} {name if name else "unknown"}'

result:

if name exists, e.g. name = 'Mike'

hello Mike

otherwise

hello unknown

Can this be done and if yes how?

回答1:

Both ternaries ("if expressions") and comprehensions ("for expressions") are allowed inside f-strings. However, they must be part of expressions that evaluate to strings. For example, key: value is a dict pair, and f"{key}: {value}" is required to produce a string.

>>> dct = {'a': 1, 'b': 2}
>>> newline = "\n"  # \escapes are not allowed inside f-strings
>>> print(f'{newline.join(f"{key}: {value}" for key, value in dct.items())}')
a: 1
b: 2

Note that if the entire f-string is a single format expression, it is simpler to just evaluate the expression directly.

>>> print("\n".join(f"{key}: {value}" for key, value in dct.items())))
a: 1
b: 2

Expressions inside format strings still follow their regular semantics. For example, a ternary may test whether an existing name is true. It will fail if the name is not defined.

>>> c, name = "Hello", ""
>>> f'{c} {name if name else "unknown"}'
'Hello unknown'
>>> del name
>>> f'{c} {name if name else "unknown"}'
NameError: name 'name' is not defined

来源：https://stackoverflow.com/questions/59956496/f-strings-formatter-including-for-loop-or-if-conditions