On 1/7/2010 2:12 AM, Phlip wrote:
On Jan 5, 10:54 pm, Benjamin Kaplan<benjamin.kap...@case.edu> wrote:{41: None}[41] ? In cases where None is a valid result, you can't use it to signal failure..Asked and answered. You change the "sentinel" in .fetch to something else.
I believe Ben Kaplan's point is that if dict slicing returns sentinel for missing keys, the slicing syntax would be unusable to differentiate {41: sentinel} and {}; if the default sentinel had been None, that would make it too easy to forget to reset the sentinel and wrote a buggy code.
But y'all keep on defending the language making your programming decisions for you!
When designing a language, there are two different ways to signal a missing key in a dict: 1) raise an exception or 2) return a sentinel. Both are useful for different purpose. One or the other must be the default behavior, and the other must give way.
Python decided that the default behavior should be raising exception and sentinel have to use the dict.get() method. Simple and clear. The other possible behavior (i.e. slicing returns a sentinel while dict.get() raises an exception) is arguably just as simple and just as clear; but python doesn't do it that way. Why? Because it's part of what makes python Python[1].
But y'all keep on defending the language making your programming decisions for you!
Indeed, the language makes decisions for you; why would you want to use a language that nags you for every possible different behavior? I want a reasonable default and I want a reasonably easy way to access the non-default behaviors.
[1] the essence of which is emboldened as the Zen, which is the broad design guideline for both python's syntax and codes written in python. Specifically, "Errors should never pass silently" because missing key indicates a possible error and "Simple is better than complex" because returning sentinel for missing keys implies the need for an interface to change the sentinel[2]. [2] or become like Java, whose HashMap cannot reliably store null and doesn't allow you to specify the sentinel.
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