[Python-Dev] if key in dict?

[Fred L. Drake, Jr.]

Skip Montanaro writes:
 > Also, for many situations, "if value in dict" will be extraordinarily
 > inefficient.  In "in" semantics are added to dicts, a corollary move will be
 > to extend this functionality to other non-dict mappings (e.g., file-based
 > mapping objects like gdbm).  Implementing "in" for them would be
 > excruciatingly slow if the LHS was "value".  To not break the rule of least
 > astonishment when people push large dicts to disk, the only feasible
 > implementation is "if key in dict".

Skip,
  Performance issues aside, I can see very valid reasons for the x in
"x in dict" to be either the key or (key, value) pair.  For this
reason, I've come to consider "x in dict" a mis-feature, though I once
pushed for it as well.  It may be easy to explain that x is just the
key, but it's not clearly the only reasonably desirable semantic.


  -Fred

--
Fred L. Drake, Jr.	  <fdrake at acm.org>
Corporation for National Research Initiatives