Python: update dict string that has placeholders?

Question

Consider this string: "{'a': A, 'b': B, 'c': 10}". Now I want to update this "string" and add new key d with let say value 20, so result would be "{'a': A, 'b': B, 'c': 10, 'd': 20}"

Normally, you could just eval string (eval or literal_eval) into dict, update the way you want and convert it back to string. But in this case, there are placeholders, which would not be recognized when evaluating.

What would be best way to update it, so old values are kept the same, but "dict-string" is updated properly?

Answer 1

This by no means a best solution but here is one approach:

import re

dict_str = "{'a': A, 'b': B, 'c': 10}"

def update_dict(dict_str, **keyvals):
    """creates an updated dict_str

        Parameters:
            dict_str (str): current dict_str
            **keyvals: variable amounts of key-values

        Returns:
            str:updated string

    """
    new_entries = ", ".join(map(lambda keyval: f"'{keyval[0]}': {keyval[1]}", keyvals.items())) # create a string representation for each key-value and join by ','
    return dict_str.replace("}", f", {new_entries}{'}'}")   # update the dict_str by removing the last '}' and add the new entries

output:

updated = update_dict(dict_str,
    d = 20,
    e = 30
)
print(updated)

{'a': A, 'b': B, 'c': 10, 'd': 20, 'e': 30}

some_dict = {
    'g': 2,
    'h': 3
}

updated = update_dict(dict_str,
    **some_dict
)
print(updated)

{'a': A, 'b': B, 'c': 10, 'g': 2, 'h': 3}