[Python-ideas] f-string "debug" conversion

Steven D'Aprano steve at pearwood.info
Wed Oct 3 04:05:39 EDT 2018

On Wed, Oct 03, 2018 at 06:57:19AM +0200, Anders Hovmöller wrote:

> debug(next=value+1)
> Still shorter than the proposed syntax 

Are we trying to emulate Perl now? *wink*

> and much more readable. 

So you say.

To me that looks like a regular function call, which calls an ordinary 
function "debug" and takes a simple keyword argument next with value 

Things which contain compiler magic should look special, not like 
ordinary function calls.

> > AIUI, keyword arguments are all supposed to be legal names/atoms, so
> > you aren't supposed to do something like this:
> > 
> > debug(**{"value+1":value+1})
> Really? That seems pretty weird to me. I’ve used that type of thing in 
> production code from time to time.

The fact that this works is, I think, an accident of implementation:

py> def spam(**kw):
...     print(kw)
py> spam(**{"value+1": 42})
{'value+1': 42}

rather than a guaranteed language feature. I can't find any relevent 
documentation on it, but I'd be very wary about relying on it.

(To be honest, I expected it to fail until I tried it.)

You certainly can't do this:

py> spam(value+1=42)
  File "<stdin>", line 1
SyntaxError: keyword can't be an expression


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