String format a JSON string gives KeyError

Question

Why does this code give a KeyError?

output_format = """ {      "File": "{filename}",      "Success": {success},      "ErrorMessage": "{error_msg}",      "LogIdentifier": "{log_identifier}"  } """  print output_format.format(filename='My_file_name',                            success=True,                            error_msg='',                            log_identifier='123') 

Error message:

KeyError: ' "File"' 

Answer 1

You need to double the outer braces; otherwise Python thinks { "File".. is a reference too:

output_format = '{{ "File": "{filename}", "Success": {success}, "ErrorMessage": "{error_msg}", "LogIdentifier": "{log_identifier}" }}' 

Result:

>>> print output_format.format(filename='My_file_name', ...                            success=True, ...                            error_msg='', ...                            log_identifier='123') { "File": "My_file_name", "Success": True, "ErrorMessage": "", "LogIdentifier": "123" } 

If, indicentally, you are producing JSON output, you'd be better off using the json module:

>>> import json >>> print json.dumps({'File': 'My_file_name', ...                   'Success': True, ...                   'ErrorMessage': '', ...                   'LogIdentifier': '123'}) {"LogIdentifier": "123", "ErrorMessage": "", "Success": true, "File": "My_file_name"} 

Note the lowercase true in the output, as required by the JSON standard.

Answer 2

As mentioned by Tudor in a comment to another answer, the Template class was the solution that worked best for me. I'm dealing with nested dictionaries or list of dictionaries and handling those were not as straightforward.

Using Template though the solution is quite simple.

I start with a dictionary that is converted into a string. I then replace all instances of { with ${ which is the Template identifier to substitute a placeholder.

The key point of getting this to work is using the Template method safe_substitute. It will replace all valid placeholders like ${user_id} but ignore any invalid ones that are part of the dictionary structure, like ${'name': 'John', ....

After the substitution is done I remove any leftovers $ and convert the string back to a dictionary.

In the code bellow, resolve_placeholders returns a dictionary where each key matches a placeholder in the payload string and the value is substituted by the Template class.

from string import Template . . . payload = json.dumps(payload) payload = payload.replace('{', '${') replace_values = self.resolve_placeholders(payload) if replace_values:     string_template = Template(payload)     payload = string_template.safe_substitute(replace_values) payload = payload.replace('${', '{') payload = json.loads(payload)