popkey() method for dictionaries?

[Fernando Pérez]

Raymond Hettinger wrote:

> Do you have use cases demonstrating the value of a default rather than an
> exception?

Sure. First, on principle: the exception option is still there, if no default 
is provided. What giving a default buys you is not having to trap the 
exception yourself. If you want a missing key to generate an exception, 
simply don't give a default and that's it. Since it doesn't change the 
existing semantics (the current form doesn't take any arguments, so there 
can't be any confusion), I think it's a good addition.

But you asked for code. Here's an example from a wrapper which needs to filter 
out a keyword dictionary given to a function before passing it downstream. It 
needs to remove keywords which won't be understood by the downstream 
function, but it knows how to make a default decision if the keyword wasn't 
given:

        # Filter out other options the original plot doesn't know
        hardcopy = popkey(keyw,'hardcopy',psargs['filename'] is not None)
        titles = popkey(keyw,'titles',0)

This uses my popkey() function, with the new method it would be simply

        # Filter out other options the original plot doesn't know
        hardcopy = keyw.pop('hardcopy',psargs['filename'] is not None)
        titles = keyw.pop('titles',0)

if my suggestion were accepted. I can always live with my popkey() function 
instead, if the crippled version is left in the language :) What I _won't_ do 
is use the crippled pop() and wrap things everywhere with explicit try/except 
blocks. In the end that only hurts readaility and creates bloat.

This is part of the Gnuplot wrappers in IPython, if you want to see the full 
code (or I can give more context). IPython lives at 
http://www-hep.colorado.edu/~fperez/ipython

> Also, discuss why similarity with dict.get() applies instead of symmetry
> with list.pop() or dict.popitem().

- list.pop: I wasn't looking when that was written :) But I could argue 
similarly that an optional default argument would be a sensible, useful 
addition. No semantic break for existing code, and it saves people setting up 
try/except traps.

- dict.popitem: this one I'm ok with not having a default value, since it is 
meant to traverse the dict in essentially random order. Adding a default 
value would be like having a dict_with_default kind of object, which is 
definitely different from a regular python dict. popitem() returns a k/v pair 
and is typically used for exhausting a dict destructively, so it makes a lot 
of sense to break at the end. 

But pop(key) is much more specific, so I think it is sensible to be able to 
control its behavior more closely. Just like dict.get(), which allows 
straight through the function interface one to control what happens to 
missing keys. Semantically pop(key) is almost like get(key), with the 
difference that it deletes the k/v entry from the dict. It seems only natural 
then that pop() would support the same default arg behavior that get() has.

Cheers,

f.