Hidden features of Python [closed]
Chaining comparison operators:
>>> x = 5>>> 1 < x < 10True>>> 10 < x < 20 False>>> x < 10 < x*10 < 100True>>> 10 > x <= 9True>>> 5 == x > 4TrueIn case you're thinking it's doing 1 < x, which comes out as True, and then comparing True < 10, which is also True, then no, that's really not what happens (see the last example.) It's really translating into 1 < x and x < 10, and x < 10 and 10 < x * 10 and x*10 < 100, but with less typing and each term is only evaluated once.
Get the python regex parse tree to debug your regex.
Regular expressions are a great feature of python, but debugging them can be a pain, and it's all too easy to get a regex wrong.
Fortunately, python can print the regex parse tree, by passing the undocumented, experimental, hidden flag re.DEBUG (actually, 128) to re.compile.
>>> re.compile("^\[font(?:=(?P<size>[-+][0-9]{1,2}))?\](.*?)[/font]", re.DEBUG)at at_beginningliteral 91literal 102literal 111literal 110literal 116max_repeat 0 1 subpattern None literal 61 subpattern 1 in literal 45 literal 43 max_repeat 1 2 in range (48, 57)literal 93subpattern 2 min_repeat 0 65535 any Nonein literal 47 literal 102 literal 111 literal 110 literal 116Once you understand the syntax, you can spot your errors. There we can see that I forgot to escape the [] in [/font].
Of course you can combine it with whatever flags you want, like commented regexes:
>>> re.compile(""" ^ # start of a line \[font # the font tag (?:=(?P<size> # optional [font=+size] [-+][0-9]{1,2} # size specification ))? \] # end of tag (.*?) # text between the tags \[/font\] # end of the tag """, re.DEBUG|re.VERBOSE|re.DOTALL)
enumerate
Wrap an iterable with enumerate and it will yield the item along with its index.
For example:
>>> a = ['a', 'b', 'c', 'd', 'e']>>> for index, item in enumerate(a): print index, item...0 a1 b2 c3 d4 e>>>References: