[Python-Dev] Draft Guide for code migration and modernation

[Skip Montanaro]

    Raymond> Pattern:    if d.has_key(k):  --> if k in d:
    Raymond> Idea:   For testing dictionary membership, use the 'in' keyword
    Raymond> instead of the 'has_key()' method.
    Raymond> Version:   2.2 or greater
    Raymond> Benefits:   The result is shorter and more readable. The style
    Raymond> becomes consistent with tests for membership in lists.  The
    Raymond> result is slightly faster because has_key requires an attribute
    Raymond> search.

Also faster (I think) because it avoids executing the expensive
CALL_FUNCTION opcode.  (Probably applies to the d.keys() part of second
pattern as well.)

    Raymond> Pattern:    if v == None  -->  if v is None:
    Raymond> Idea:   Since there is only one None object, it can be tested
    Raymond> with identity. 
    Raymond> Version:   Any
    Raymond> Benefits:   Identity tests are slightly faster than equality
    Raymond> tests. Also, some object types may overload comparison to be
    Raymond> much slower (or even break). 
    Raymond> Locating: grep '== None' or grep '!= None'

Also:

    if v: --> if v is None

where appropriate (often when testing function arguments that default to
None).  This may change semantics though and has to be undertaken with some
care.

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