Write Pythonic Code Like a Seasoned Developer Transcripts
Chapter: Pythonic Loops
Lecture: Loops have an else block, don't use it
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One peculiar feature of Python loops is that they have an "else" block, first of all, do you know that the "while" loop
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and the "for...in" loop have "else" clauses that you can attach to them? Second, can you remember the order in which these happen,
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what triggers the "else" block to run versus it not running? Let's have a look, so here we have two "while" loops, one "while" loop runs to completion;
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the other "while" loop breaks out early, let's just run to see the output. Cool, so here we have 5 dots and here we have 4 dots,
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that means we've run through one all the way to the end, the other we stopped a little bit early.
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So, there is nothing special about that, but let's see about the "else" clause here,
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so let's say "for all loops" we can say "else", so "do the loop, else", we'll just put a message to know that it ran or didn't in the "else" clause,
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so we'll write something like this, "in the else clause of the whole loop",
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and down here we'll say "else", in the "else" clause of the early break loop, which one is going to run? Let's find out.
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OK, so if we run through the entire loop, and we go all the way to the end, this becomes False, and then "else", we run this.
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On the other hand, if we go through here and we never go to a case where this is no longer True
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but we break out early instead, we don't run the "else" clause. I am sure there is a few good uses for this, somewhere,
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it doesn't seem like a language feature that's worth it to me, but my recommendation for Pythonic code is: Do not use the "else" clause on loops.
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Yeah, here is a whole language feature, I say don't use it, now you might say, "Michael, is that really Pythonic? I mean it is a language feature,
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who would say "Don't use a language feature that was put in there by Guido Van Rossum, the creator of Python himself?"" let's see.
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Back in 2009, somebody asked about the "for/while/else" without "break" or "return" clauses and Guido dropped in and the guy said,
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"Well, I am not sure that this is a great choice of words", and Guido dropped in and said, "You know what,
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I would not have this feature at all if I had to do it over". "I would not put the "else" clause on loops in the language, period."
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That to me is a pretty strong statement that you know, we don't really need this, there are many successful languages that don't have an "else" clause,
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I think the "else" clause is confusing, Guido thinks it's probably not the best to put it in there, let's all agree to avoid it.
2:27
Right, so we saw that we have "else" clauses here, if you break out the loop early in this case, the "else" clause does not run,
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if we run the loop to completion, basically to a False state, then it will run. "I would not have put the "else" clause in the language at all
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if I had to do it over", to me it sounds like non-Pythonic.