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Can anyone please help me understand this code snippet, from http://garethrees.org/2007/05/07/python-challenge/ Level2

>>> import urllib
>>> def get_challenge(s):
...     return urllib.urlopen('http://www.pythonchallenge.com/pc/' + s).read()
...
>>> src = get_challenge('def/ocr.html')
>>> import re
>>> text = re.compile('<!--((?:[^-]+|-[^-]|--[^>])*)-->', re.S).findall(src)[-1]
>>> counts = {}
>>> for c in text: counts[c] = counts.get(c, 0) + 1
>>> counts

http://garethrees.org/2007/05/07/python-challenge/

re.compile('<!--((?:[^-]+|-[^-]|--[^>])*)-->', re.S).findall(src)[-1] why we have [-1] here what's the purpose of it? is it Converting that to a list? **

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2 Answers 2

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Yes. re.findall() returns a list of all the matches. Have a look at the documentation.

re.findall(pattern, string, flags=0)

Return all non-overlapping matches of pattern in string, as a list of strings. The string is scanned left-to-right, and matches are returned in the order found. If one or more groups are present in the pattern, return a list of groups; this will be a list of tuples if the pattern has more than one group. Empty matches are included in the result unless they touch the beginning of another match.

When calling [-1] on the result, the first element from the end of the list is accessed.

For example;

>>> a = [1,2,3,4,5]
>>> a[-1]
5

And also:

>>> re.compile('.*?-').findall('-foo-bar-')[-1]
'bar-'
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5 Comments

I got findall is for matching, i'm fine with it. However what makes someone insert [-1 ] at the end of re.compile('<!--((?:[^-]+|-[^-]|--[^>])*)-->', re.S).findall(src)[-1] ? Couldn't understand his intention.
It's simply array access - don't be fooled by the negative index! :)
But by putting [1] it's working fine! :/ and Could you also please explain what's going here in this line? for c in text: counts[c] = counts.get(c, 0) + 1
That line is building a dictionary of the frequencies of each character in text. The zero in get(c, 0) is the default that is returned if c was not previously in the dictionary, but otherwise the count is simply incremented by 1 for each occurance.
I tried this pattern (<!--((?:[^-]+|-[^-]|--[^>])*)-->, r.S ), r.S is for all match(dot All) right? in regex interpreter to check for matches inside the page source text content ('!@#$%^&*@#$%#$^%#$^$#%^#') of the challenge . It gives a match only when i remove <!-- --> otherwise it gives null. why is that?
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It's already a list. And if you have a list myList, myList[-1] returns the last element in that list.

Read this: https://docs.python.org/2/tutorial/introduction.html#lists.

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5 Comments

Thanks for the quick reply, so does putting a [] after an expression converts that into a list?? Bec if i put [1] it works the same way for [-1] but not for [2].
It was already a list. [] just retrieves an element or slice from the list. Changing the number in there is just going to change which element it accesses. Check out the page I linked to for more examples about list indexing and slices.
I got confused because if i try to print (text) it goes on like a big chunk, but if i keep [1]/[-1] at the end , it prints line by line.
Yeah! i got it now. Very eloquent description :)
I tried this pattern ((<!--((?:[^-]+|-[^-]|--[^>])*)-->, r.S ), r.S is for all match(dot All) right? in regex interpreter to check for matches inside the page source text content ('!@#$%^&*@#$%#$^%#$^$#%^#') of the challenge . It gives a match only when i remove <!-- --> otherwise it gives null. why is that?

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