Code Explaination: Spelling correction code

[Steven Bethard]

Drew wrote:
> On Apr 11, 11:27 pm, Steven Bethard <steven.beth... at gmail.com> wrote:
>> Drew wrote:
>>> def known_edits2(word):
>>>     return set(e2 for e1 in edits1(word) for e2 in edits1(e1) if e2 in
>>> NWORDS)
>>
>> This is the same as:
>>
>>      result = set()
>>      for e1 in edits1(word):
>>          for e2 in edits1(e1):
>>              if e2 in NWORDS:
>>                  result.add(e2)
>>      return result
>>
>> The thing between the ``set(`` and ``)`` is called a generator
>> comprehension if you'd like to look into it further.
> 
> Thanks for the response. I'm somewhat familiar with generator/list
> comprehension but was unsure how multiple statements were evaluated
> when chained together. From your explanation, I'm assuming they are
> evaluated from the "inside out" rather than left to right or right to
> left.
> 
> Does the mean that the comprehension on the inside is always evaluated
> first?

Not really (at least for the most literal interpretation of ``evaluated 
first``). I find it easiest to think of translating them into regular 
for loops by adding the appropriate indentation.

Starting with:

(e2 for e1 in edits1(word) for e2 in edits1(e1) if e2 in NWORDS)

Adding newlines:

(e2
  for e1 in edits1(word)
  for e2 in edits1(e1)
  if e2 in NWORDS)

Adding indentation:

(e2
  for e1 in edits1(word)
      for e2 in edits1(e1)
          if e2 in NWORDS)

Moving the add/append to the bottom:

for e1 in edits1(word)
     for e2 in edits1(e1)
         if e2 in NWORDS
             e2

Adding the remaining boiler-plate:

result = set()
for e1 in edits1(word):
     for e2 in edits1(e1):
         if e2 in NWORDS:
             result.add(e2)


So multiple for- and if-expressions are evaluated in the same order that 
they would normally be in Python, assuming the proper whitespace was added.

HTH,

STeVe