On 5/13/2018 3:22 PM, Ken Kundert wrote:
Please do not double post.
I am seeing an unexpected difference between the behavior of the string
format method and f-strings.
Read
https://docs.python.org/3/reference/lexical_analysis.html#formatted-string-literals
carefully.
Here is an example:
import sys, os
from inform import error, os_error
class mydict(dict):
def __format__(self, template):
print('Template:', template)
return ', '.join(template.format(v, k=k, v=v) for k, v in
self.items())
d = mydict(bob='239-8402', ted='371-8567', carol='891-5810',
alice='552-2219')
print('Using format():')
print('Email: {0:{{k}}: {{v}}}'.format(d))
print()
print('Using f-string:')
print(f'Email: {d:{{k}} {{v}}}')
print()
print('Using f-string:')
print(f'Email: {d:{{k}} {{v}}}', k=6, v=9)
It generates the following response:
Using format():
Template: {k}: {v}
Email: bob: 239-8402, ted: 371-8567, carol: 891-5810, alice: 552-2219
Using f-string:
Traceback (most recent call last):
File "tryit", line 18, in <module>
print(f'Email: {d:{{k}} {{v}}}')
NameError: name 'k' is not defined
This is what I expected.
Essentially I am using a format string as the template that indicates
how to format each member of a dictionary, {{k}} should interpolate the
key and {{v}} interpolates the value. This format string is embedded
inside another format string, so the braces are doubled up so that they
will be ignored by the outer format string.
"The parts of the string outside curly braces are treated literally,
except that any doubled curly braces '{{' or '}}' are replaced with the
corresponding single curly brace. " note 'outside'
This idea seems to work okay when using the format() method. You can see
I added a print statement inside __format__ that shows that the method
is being called.
However, trying the same idea with f-strings results in a NameError. It
appears that the escaping does not work when used within the template.
It appears the error occurs before __format__ is called (there is no
output from the print function).
Does anybody know why the format() method would work in this case but
the f-string would not?
All names in the expression are resolved in the local namespace of the f
string. There are other differences. Nesting can only be one level deep.
Is this a bug in f-strings?
Not to me.
--
Terry Jan Reedy
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