use .format() in a string in two steps

Question

I have a string in which I want to replace some variables, but in different steps, something like:

my_string = 'text_with_{var_1}_to_variables_{var_2}'
my_string.format(var_1='10')
### make process 1
my_string.format(var_2='22')

But when I try to replace the first variable I get an Error:

KeyError: 'var_2'

How can I accomplish this?

Edit: I want to create a new list:

name = 'Luis'
ids = ['12344','553454','dadada']
def create_list(name,ids):
    my_string = 'text_with_{var_1}_to_variables_{var_2}'.replace('{var_1}',name)
    return [my_string.replace('{var_2}',_id) for _id in ids ]

this is the desired output:

['text_with_Luis_to_variables_12344',
 'text_with_Luis_to_variables_553454',
 'text_with_Luis_to_variables_dadada']

But using .format instead of .replace.

Not quite sure what you're trying to do here. Are you going to use `my_string` somehow in between those two steps? — Daniel Roseman, Aug 28 '16 at 21:38
Can you explain what it is you are trying to accomplish exactly? This seems like an [XY](http://mywiki.wooledge.org/XyProblem) problem. — idjaw, Aug 28 '16 at 21:44
Possible duplicate of [partial string formatting](http://stackoverflow.com/questions/11283961/partial-string-formatting) — Delgan, Aug 28 '16 at 21:48
@DanielRoseman I'm thinking in two steps because the second variable is a list. I updated the question. — Luis Ramon Ramirez Rodriguez, Aug 29 '16 at 02:29
@idjaw Interesting link :). Yeah, I could be a XY problem. But I'm curious about using .format() in more than one step. — Luis Ramon Ramirez Rodriguez, Aug 29 '16 at 02:31

Moinuddin Quadri · Accepted Answer · 2016-08-28T22:17:53.310

In simple words, you can not replace few arguments with format {var_1}, var_2 in string(not all) using format. Even though I am not sure why you want to only replace partial string, but there are few approaches that you may follow as a workaround:

Approach 1: Replacing the variable you want to replace at second step by {{}} instead of {}. For example: Replace {var_2} by {{var_2}}

>>> my_string = 'text_with_{var_1}_to_variables_{{var_2}}'
>>> my_string = my_string.format(var_1='VAR_1')
>>> my_string
'text_with_VAR_1_to_variables_{var_2}'
>>> my_string = my_string.format(var_2='VAR_2')
>>> my_string
'text_with_VAR_1_to_variables_VAR_2'

Approach 2: Replace once using format and another using %.

>>> my_string = 'text_with_{var_1}_to_variables_%(var_2)s'
# Replace first variable
>>> my_string = my_string.format(var_1='VAR_1')
>>> my_string
'text_with_VAR_1_to_variables_%(var_2)s'
# Replace second variable
>>> my_string  = my_string % {'var_2': 'VAR_2'}
>>> my_string
'text_with_VAR_1_to_variables_VAR_2'

Approach 3: Adding the args to a dict and unpack it once required.

>>> my_string = 'text_with_{var_1}_to_variables_{var_2}'
>>> my_args = {}
# Assign value of `var_1`
>>> my_args['var_1'] = 'VAR_1'
# Assign value of `var_2`
>>> my_args['var_2'] = 'VAR_2'
>>> my_string.format(**my_args)
'text_with_VAR_1_to_variables_VAR_2'

Use the one which satisfies your requirement. :)

score 1 · Answer 2 · answered Aug 28 '16 at 21:44

1

Do you have to use format? If not, can you just use string.replace? like

my_string = 'text_with_#var_1#_to_variables_#var2#'
my_string = my_string.replace("#var_1#", '10')
###
my_string = my_string.replace("#var2#", '22')

answered Aug 28 '16 at 21:44

Peter

775
1
6
12

score 0 · Answer 3 · answered Dec 19 '22 at 09:01

0

following seems to work now.

s = 'a {} {{}}'.format('b')
print(s) # prints a b {}
print(s.format('c')) # prints a b c

answered Dec 19 '22 at 09:01

sakitha_sen

27
5

use .format() in a string in two steps

3 Answers3