How should a variable be passed within format() in Python?

Question

I am seeking to be able to use variables within the format() parentheses, in order to parameterize it within a function. Providing an example below:

sample_str = 'sample_str_{nvars}'
nvars_test = 'apple'
sample_str.format(nvars = nvars_test)  #Successful Result: ''sample_str_apple''

But the following does not work -

sample_str = 'sample_str_{nvars}'
nvars_test_2 = 'nvars = apple'
sample_str.format(nvars_test_2) # KeyError: 'nvars'

Would anyone know how to do this? Thanks.

try: {0}. As your add dictionary in first example, it works. In second example, you add list, so you need to put position — Valijon
– Valijon, Commented Sep 27, 2018 at 19:46
I wanted to mention the popular advice "keep data out of your variable names", but actually, this is more like "keep variable names out of your data"! In any case, variable names and data live in orthogonal dimensions, it's best to try not to mix them up too much. — wim
– wim, Commented Sep 27, 2018 at 19:50
@wim This isn't any worse than defining a function with documented parameter names. — chepner
– chepner, Commented Sep 27, 2018 at 19:51
Don't answer your question in the question itself. Either post a proper answer, or delete the question. — chepner
– chepner, Commented Sep 27, 2018 at 19:57
@chepner Yes it is. The line nvars_test_2 = 'nvars = apple' has a variable name (nvars) hidden in a string of data (the object 'nvars = apple'). — wim
– wim, Commented Sep 27, 2018 at 20:02

Ashley · Accepted Answer · 2018-09-27 20:00:06Z

1

Many thanks for guidance. I did a bit more searching. For anyone who may run into the same problem, please see examples here: https://pyformat.info

sample_str = 'sample_str_{nvars}'
nvars_test_2 = {'nvars':'apple'} 
sample_str.format(**nvars_test_2)  #Successful Result: ''sample_str_apple''

answered Sep 27, 2018 at 20:00

Ashley

855 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

alex · Accepted Answer · 2018-09-27 20:00:53Z

First, I'd recommend checking out the string format examples.

Your first example works as expected. From the documentation, you are permitted to actually name the thing you are passing into {}, and then pass in a same-named variable for str.format():

'Coordinates: {latitude}, {longitude}'.format(latitude='37.24N', longitude='-115.81W')
# returns 'Coordinates: 37.24N, -115.81W'

Your second example doesn't work because you are not passing a variable called nvars in with str.format() - you are passing in a string: 'nvars = apple'.

sample_str = 'sample_str_{nvars}'
nvars_test_2 = 'nvars = apple'
sample_str.format(nvars_test_2) # KeyError: 'nvars'

It's a little more common (I think) to not name those curly-braced parameters - easier to read at least.

print('sample_str_{}'.format("apple")) should return 'sample_str_apple'.

Michael Ekoka · Accepted Answer · 2018-09-27 20:27:54Z

0

If you're using Python 3.6 you also have access to Python's formatted string literals.

>>> greeting = 'hello'
>>> name = 'Jane'
>>> f'{greeting} {name}'
'hello Jane'

Note that the literal expects the variables to be already present. Otherwise you get an error.

>>> f'the time is now {time}'
NameError: name 'time' is not defined

answered Sep 27, 2018 at 20:27

Michael Ekoka

20.3k12 gold badges83 silver badges83 bronze badges

Collectives™ on Stack Overflow

How should a variable be passed within format() in Python?

3 Answers 3

Comments

Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Related