Underscore as variable name in Python

Question

def search(values):
    "Using depth-first search and propagation, try all possible values."
    if values is False:
        return False ## Failed earlier
    if all( len( values[s]) == 1 for s in squares): 
        return values ## Solved!
    ## Chose the unfilled square s with the fewest possibilities
    _,s = min( (len( values[s]), s) 
                for s in squares 
                if len(values[s]) > 1
            )
    return some( search( assign( values.copy(), s, d)) 
                for d in values[s]
            )

Answer 1

Yep, _ is a traditional name for "don't care" (which unfortunately clashes with its use in I18N, but that's a separate issue;-). BTW, in today's Python, instead of:

_,s = min( (len( values[s]), s) 
            for s in squares 
            if len(values[s]) > 1
        )

you might code

s = min((s for s in squares if len(values[s])>1), 
        key=lambda s: len(values[s]))

(not sure what release of Python Peter was writing for, but the idiom he's using is an example of "decorate-sort-undecorate" [[DSU]] except with min instead of sort, and in today's Python the key=optional parameter is generally the best way to do DSU;-).