Unexpected '{' in field name when doing string formatting

Question

I'm trying to write a small script that will automate some PHP boilerplate that I need to write. It should write a copy of the string code to the output file with the various replacement fields filled in for each dict in the fields list.

However, I'm getting the error:

Traceback (most recent call last):
    File "writefields.py", line 43, in <module>
        formatted = code.format(**field)
ValueError: unexpected '{' in field name

As far as I can tell, there are no extra braces in either the replacement fields or the dicts that should be causing issues, so any help would be appreciated.

code = '''
// {label}
add_filter( 'submit_job_form_fields', 'frontend_add_{fieldname}_field' );
function frontend_add_{fieldname}_field($fields) {
    $fields['job']['job_{fieldname}'] = array(
        'label' => __('{label}', 'job_manager'),
        'type' => 'text',
        'required' => {required},
        'priority' => 7,
        'placeholder' => '{placeholder}'
    );
    return $fields;
}
add_filter( 'job_manager_job_listing_data_fields', 'admin_add_{fieldname}_field' );
function admin_add_{fieldname}_field( $fields ) {
  $fields['_job_{fieldname}'] = array(
    'label'       => __( '{label}', 'job_manager' ),
    'type'        => 'text',
    'placeholder' => '{placeholder}',
    'description' => ''
  );
  return $fields;
}
'''

fields = [
    {
        'fieldname': 'salary',
        'label': 'Salary ($)',
        'required': 'true',
        'placeholder': 'e.g. 20000',
    },
    {
        'fieldname': 'test',
        'label': 'Test Field',
        'required': 'true',
        'placeholder': '',
    }
]

with open('field-out.txt', 'w') as f:
    for field in fields:
        formatted = code.format(**field)
        f.write(formatted)
        f.write('\n')

Martijn Pieters · Accepted Answer · 2021-01-09 16:01:19Z

You need to double any { or } that are not part of a formatting placeholder. For example, you have:

function admin_add_{fieldname}_field( $fields ) {
    [....]
}

in the string. The { at the end of the first line and } on the last are not part of a placeholder. Replace them with {{ and }} respectively:

function admin_add_{fieldname}_field( $fields ) {{
    [....]
}}

Doubling up those curly braces escapes them; the final output will contain single { and } characters again.

In the full string you used, that'd be:

code = '''
// {label}
add_filter( 'submit_job_form_fields', 'frontend_add_{fieldname}_field' );
function frontend_add_{fieldname}_field($fields) {{
    $fields['job']['job_{fieldname}'] = array(
        'label' => __('{label}', 'job_manager'),
        'type' => 'text',
        'required' => {required},
        'priority' => 7,
        'placeholder' => '{placeholder}'
    );
    return $fields;
}}
add_filter( 'job_manager_job_listing_data_fields', 'admin_add_{fieldname}_field' );
function admin_add_{fieldname}_field( $fields ) {{
  $fields['_job_{fieldname}'] = array(
    'label'       => __( '{label}', 'job_manager' ),
    'type'        => 'text',
    'placeholder' => '{placeholder}',
    'description' => ''
  );
  return $fields;
}}
'''

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