python: dots in the name of variable in a format string

Question

Say I've got a dictionary with dots in the name of fields, like {'person.name': 'Joe'}. If I wanted to use this in str.format, is it possible?

My first instinct was

'Name: {person.name}'.format(**{'person.name': 'Joe'})

but this would only work if my dict were shaped like

{'person':{'name':Joe}}

The relevant manual docs section doesn't mention anyway of escaping the dot.

(Sidenote: I thought that generally

def func(**kw): print(kw)
func(**{'a.b': 'Joe'})

would cause an error, but the **-expanded function call seems to work even if they're not valid identifiers! It does error out on non-strings though. o_O)

@RafeKettler: Yes. That doesn't work because you're escaping it for the string, not the format specifier. — nfirvine
– nfirvine, Commented Oct 29, 2011 at 6:18
Possible duplicate of How to use a dot in Python format strings? — Amir Ali Akbari
– Amir Ali Akbari, Commented Sep 18, 2016 at 7:59

Michael Hoffman · Accepted Answer · 2011-10-28 21:10:12Z

8

'Name: {0[person.name]}'.format({'person.name': 'Joe'})

answered Oct 28, 2011 at 21:10

Michael Hoffman

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4 Comments

jathanism Over a year ago

Close but that would only work if you're passing a dictionary with a single key, one for each positional replacement. For example this would fail: 'Name: {0[person.name]} Age: {1[person.age]}'.format({'person.name': 'Joe', 'person.age': 42})

Michael Hoffman Over a year ago

But you can repeat 0 as many times as is necessary. This works fine: 'Name: {0[person.name]} Age: {0[person.age]}'.format({'person.name': 'Joe', 'person.age': 42})

jathanism Over a year ago

Awesome, I was not aware you could do that and I have learned something. Thanks! :)

nfirvine Over a year ago

Note that since 2.7 you can omit the integer indices if it's just the same order as in the string: 'Name: {[person.name]}'.format({'person.name': 'Joe'}) yields 'Name: Joe'. It would still require reusing 0 for subsequent access. My only problem is that it's kind of ugly. I guess I could just mangle it to 'person_age' if I needed to make sure.

Ski · Accepted Answer · 2015-02-05 10:38:53Z

7

I had similar issue and I solved it by inheriting from string.Formatter:

import string

class MyFormatter(string.Formatter):
    def get_field(self, field_name, args, kwargs):
        return (self.get_value(field_name, args, kwargs), field_name)

however you can't use str.format() because it's still pointing to old formatter and you need to go like this

>>> MyFormatter().vformat("{a.b}", [], {'a.b': 'Success!'})
'Success!'

answered Feb 5, 2015 at 10:38

Ski

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Comments

Sven Marnach · Accepted Answer · 2011-10-28 21:02:55Z

5

One way to work around this is to use the old % formatting (which has not been deprecated yet):

>>> print 'Name: %(person.name)s' % {'person.name': 'Joe'}
Name: Joe

answered Oct 28, 2011 at 21:02

Sven Marnach

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Comments

John Trammell · Accepted Answer · 2023-09-22 19:03:43Z

0

f-strings handle this cleanly (I realize they were not available at the time of the original post):

>>> x = {'person.name': 'Joe'}
>>> print(f"Name: {x['person.name']}")
Name: Joe

answered Sep 22, 2023 at 19:03

John Trammell

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