Behaviour of pop() for dictionaries

[Terry Reedy]

On 6/14/2021 5:18 PM, BlindAnagram wrote:

> I believe that consistency in how methods common to different types work 
> is useful since it adds to the coherence of the language as a whole and 
> avoids the need to remember special cases.

Each collection class *is* a special case, and pop has to be adjusted to 
each.  However, you seem to have missed an essential commonality.

Lists and dicts are both subscripted classes.  So the basic API is 
col.pop(sub), which removes and returns the sub item, whereas col[sub] 
leaves and returns.

Lists have a special index, -1, the most commonly used, so that is the 
default.  In fact, when I proposed list.pop(), I only proposed that, as 
I wanted pop to be the inverse of append, so a list could be used as a 
stack.

Bad list subscripts are an error (unless one is slicing), whereas where 
dicts allow a default (when subscripted with the get method).  Hence the 
optional default only for dicts.

At one time, dicts, like sets, were unordered collections (of functional 
item pairs). dict.pop(), with no arg, could have been used to return a 
random 2-ple, but Guido is generally against having return types depend 
on arguments. So a new popxxx name was added.  Note that deques have a 
popleft in addition to pop (right).

-- 
Terry Jan Reedy