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

Question

If I write in Python:

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

I get:

3, 3.14, 7, 8 Traceback (most recent call last):   File "./funky.py", line 186, in <module>     print('{n}, {k:.2f}, {p[a]}, {p[b]}'.format(**data)) KeyError: 'k' 

Instead of an error message, how can I get Python to more gracefully format the None's and non existent fields?

To give an example, I would like to see in the output something more like:

3, 3.14, 7, 8 3, ~, 7, ~ 

Ideally, of course, I would like to be able to specify the string used instead of those missing values.

Answer 1

The recommendation in PEP 3101 is to subclass Formatter:

import string class 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: raise  fmt=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, 8 del data['k'] data['p']['b'] = None print(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.

Answer 2

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'