See https://bugs.python.org/issue36774.
On 10/2/18 8:27 PM, Eric V. Smith wrote:
This idea was proposed to me at the core sprints last month by Larry
Hastings. I've discussed it with a few people, who seem generally
positive about it, and we've tweaked it a little bit. I've spent some
time implementing it, and I think it's doable. I thought I'd post it
here for any additional feedback.
Here’s the idea: for f-strings, we add a !d conversion operator, which
is superficially similar to !s, !r, and !a. The meaning of !d is:
produce the text of the expression (not its value!), followed by an
equal sign, followed by the repr of the value of the expression. So:
value = 10
s = 'a string!'
print(f'{value!d}')
print(f'next: {value+1!d}')
print(f'{s!d}')
produces:
value=10
next: value+1=11
s='a string!'
I’m not proposing this for str.format(). It would only really make sense
for named arguments, and I don’t think
print('{value!d}'.format(value=value) is much of a win.
The result is a string, so if you really wanted to, you could use a
string formatting spec. So:
print(f'*{value!d:^20}*'
would produce:
* value=10 *
Although I don’t think that would be very useful in general.
The mnemonic is !d for “debugging”. I’d wanted to use !=, because
there’s an equal sign involved in the result, but = is the one character
that can’t be used after ! (it’s “not equal” in expressions, and
f-strings look specifically for that case). I also mentioned !!, but I
think I prefer !d as being less confusing.
This would be used in debugging print statements, that currently end up
looking like:
print(f'value={value!r}')
and would now be:
print(f'{value!d}')
There have been discussions about ways to specify str() vs. repr(),
using characters other than '=', adding spaces, etc. But they all end up
over-complicating what should be a simple tool, not a Swiss Army knife.
Thoughts?
Eric
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