How to get Python to gracefully format None and non-existing fields [duplicate]

The recommendation in PEP 3101 is to subclass Formatter:

import stringclass PartialFormatter(string.Formatter):    def __init__(self, missing='~~', bad_fmt='!!'):        self.missing, self.bad_fmt=missing, bad_fmt    def get_field(self, field_name, args, kwargs):        # Handle a key not found        try:            val=super(PartialFormatter, self).get_field(field_name, args, kwargs)            # Python 3, 'super().get_field(field_name, args, kwargs)' works        except (KeyError, AttributeError):            val=None,field_name         return val     def format_field(self, value, spec):        # handle an invalid format        if value==None: return self.missing        try:            return super(PartialFormatter, self).format_field(value, spec)        except ValueError:            if self.bad_fmt is not None: return self.bad_fmt               else: raisefmt=PartialFormatter()data = {'n': 3, 'k': 3.141594, 'p': {'a': '7', 'b': 8}}print(fmt.format('{n}, {k:.2f}, {p[a]}, {p[b]}', **data))# 3, 3.14, 7, 8del data['k']data['p']['b'] = Noneprint(fmt.format('{n}, {k:.2f}, {p[a]:.2f}, {p[b]}', **data))# 3, ~~, !!, ~~

As set up, it will print ~~ if a field or attribute is not found and !! if an invalid format is used given the field value. (Just use None for the keyword argument bad_fmt if you want the default of a value error raised.)

To handle missing keys, you need to subclass both get_field to catch the KeyError or AttributeError and format_field to return a default value for the missing key.

Since you are catching format_field errors, you can catch a bad format field as well by catching the ValueError from the superclass.

If you're able to do the formatting separately you could use Template.safe_substitute which gracefully handles missing values:

>>> from string import Template>>> t = Template("$a $b $c")>>> t.safe_substitute(a=3)'3 $b $c'

The str.format() method doesn't give you a direct method to handle missing keys or replace values.

You can add a layer of indirection; pass in a mapping that handles missing and None values, and alter the format to use just that argument:

class PlaceholderFormatValue():    def __format__(self, spec):        return '~'    def __getitem__(self, name):        # handle further nested item access        return selfclass formatting_dict(dict):    def __getitem__(self, name):        value = self.get(name)        if isinstance(value, dict):            # rewrap nested dictionaries to handle missing nested keys            value = type(self)(value)        return value if value is not None else PlaceholderFormatValue()print('{0[n]}, {0[k]:.2f}, {0[p][a]}, {0[p][b]}'.format(formatting_dict(data)))

Now all slots refer to positional argument 0, which is treated like a dictionary, but key lookups always succeed and both missing values and None are replaced by a placeholder value.

Here the PlaceholderFormatValue() ensures that regardless of what the format spec gives, the value can be interpolated into the format. This makes {0[k]:.2f} work, for example.

By wrapping any dict values and having PlaceholderFormatValue handle item access, the above can also handle failure to provide nested keys or whole dictionaries:

>>> data = {'n': 3, 'k': 3.141594, 'p': {'a': 7, 'b': 8}}>>> del data['k']>>> data['p']['b'] = None>>> print('{0[n]}, {0[k]:.2f}, {0[p][a]}, {0[p][b]}'.format(formatting_dict(data)))3, ~, 7, ~>>> del data['p']['a']>>> print('{0[n]}, {0[k]:.2f}, {0[p][a]}, {0[p][b]}'.format(formatting_dict(data)))3, ~, ~, ~>>> del data['p']>>> print('{0[n]}, {0[k]:.2f}, {0[p][a]}, {0[p][b]}'.format(formatting_dict(data)))3, ~, ~, ~