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I have a string templ with variable part that I have to convert with local variables. Variable part of string have a dictionary cur_cols_dct from which I want to get a single value. Key of a dictionary is also a variable: col

templ = "COMMENT IS '{cur_cols_dct[col]}'" # string with variable part
cur_cols_dct = {'art_id':'ID of article'}  # dictionary
col = 'art_id'                             # variable for dictionary key

I have to get string with a single value from a dictionary:

COMMENT IS 'ID of article'

I can use f-string for this goal and it works:

print (f"COMMENT IS '{cur_cols_dct[col]}'")

But when I try to use format(locals()) it doesn't work, because Python can't recognize that 'col' is not a literal but a variable:

print(templ.format(**locals()))

I get an error: KeyError: 'col'

But I can't use f-string in this case because I define templ before I get cur_cols_dct and col variables.

Is there any way to get value from dictionary when both dictionary and key value are variables by using format(**locals())? Or what can I use instead of format?

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    **locals() doesn't include global variables. Commented Jun 21, 2024 at 20:49
  • Why do you want to define templ before cur_cols_dct? Why not do it afterwards with an f-string just as you said? Commented Jun 21, 2024 at 20:50
  • @quamrana It looks like he wants to define the template once, then use it repeatedly as the variables it references change. Commented Jun 21, 2024 at 20:51
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    The problem with what you're trying to do is that format() just does simple variable replacement, it doesn't evaluate expressions like f-strings do. cur_cols_dct[col] is an expression. Commented Jun 21, 2024 at 20:51
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    why not keep it as templ = "COMMENT IS '{}'" and later do directly templ.format(cur_cols_dct[col]) or templ.format(any_other_value) without using locals(). You can even use it in loop - for value in some_data: print(templ.format(valuie)) - but if you want to do something more complex then maybe use templates jinja Commented Jun 21, 2024 at 21:18

3 Answers 3

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You're attempting to use str.format as if it were a fully functional template language, which it is not. However, you can perform the details while passing arguments to it.

template = "COMMENT IS '{comment}'"
cur_cols_dct = {'art_id': 'ID of article'}
col = 'art_id'
print(template.format(comment=cur_cols_dct[col]))

COMMENT IS 'ID of article'

Here is the documentation for str.format()
https://docs.python.org/3/library/stdtypes.html#str.format

There is a string.Template class in the standard library, but I've only used django templates in python.

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3 Comments

Yes, thank you, I knew that I can do it like this, but in real I have more than one variable in my string and using locals could be a better solution. I also can set a variable comment before format and still use locals, but I have to do it every time before I use this template. Probably I will do it if I won't find something more elegant
I have a working demo of using the original template to generate another template which then prints the desired result, but I didn't post it because you don't know, in general, how many levels of indirection you need. You do know you need a comment, and you can write a function that generates the expected context for your template.
Would you consider it more elegant to say template.format(generate_context(**locals()))? I can edit this to look like that -- it's the same thing really.
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Just do something like:

def format_template(cur_cols_dct, col):
    return f"COMMENT IS '{cur_cols_dct[col]}'"

print(format_template(some_dict, some_col))
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Comments

-2 
eval(f'f"""{templ}"""')

Or as a function:

def fstr(template):
    return eval(f'f"""{template}"""')

All credit to kadee here. (The function as written in the answer doesn't work for your f-string containing quotes--it needs the multiline """ string delimiter. I've submitted an edit to the answer)

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6 Comments

why would you do this?
Oh wow, that is simultaneously brilliant and horrifying. Does it work with ast.literal_eval? It doesn't seem to have access to the caller's locals when I test this.
@juanpa.arrivillaga because it accomplishes what OP wants without them needing to modify their code. I'm just proposing one possible method, OP is free to choose any implementation
@KennyOstrom no, it doesn't
reading the linked answer, it apparently only allows the fstring to reference global variables, but we know OP wants locals()
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