How to use a dot in Python format strings?

Question

I want to format a string and be able to use the dot operator, so that I can construct template strings containing e.g. {user.name}, {product.price}.

I tried this:

'Hello {user.name}'.format( {'user': { 'name': 'Markus' } } )
KeyError: 'user'

'Hello {user.name}'.format( **{'user': { 'name': 'Markus' } } )
AttributeError: 'dict' object has no attribute 'name'

Is there a way to do it?

Answer 1

Python dict objects are unfortunately not attribute accessible (i.e. with the dot notation) by default. So you can either resign yourself to the uglier brackets notation:

'Hello {user[name]}'.format( **{'user': { 'name': 'Markus' } } )

Or you can wrap your data in a dot-accessible object. There are a handful of attribute-accessible dictionary classes you can install from PyPI, such as stuf.

from stuf import stuf

'Hello {user.name}'.format( **stuf({'user': { 'name': 'Markus' } }) )

I tend to keep my collections in stuf objects so that I can easily access them by attribute.

Answer 2

The minimal change is to use square brackets in your template, rather than a period:

              # v Note
>>> 'Hello {user[name]}'.format(**{'user': {'name': 'Markus'}})
'Hello Markus'

Alternatively, put objects that actually have that attribute in the dictionary, e.g. a custom class or collections.namedtuple:

>>> class User(object):
    def __init__(self, name):
        self.name = name


>>> 'Hello {user.name}'.format(**{'user': User('Markus')})
'Hello Markus'

Note also that if you're writing out the literal you can just use a keyword argument:

>>> 'Hello {user.name}'.format(user=User('Markus'))
'Hello Markus'

Answer 3

Flatten the dictionary using comprehension....

 def flatten_dict(dd, separator='>', prefix=''):
  return {
    prefix + separator + k if prefix else k : v
    for kk, vv in dd.items()
    for k, v in flatten_dict(vv, separator, kk).items()
  } if isinstance(dd, dict) else { prefix : dd }

A call like this:

x = { 'A':1, 'B':{'C':2}}
y = flatten_dict(x)

Produces:

y = { 'A':1, 'B>C':2}}