Programming Forums > Script Programming > Python

hydroxide · May 23rd, 2006, 9:16 AM

(Gnaaah... take two)

Quote:

Originally Posted by DaWei

It wasn't directed at you, Arevos, it was directed at the rewriting of my code...

I apologise for the overbrevity of my comment. As Arevos has suggested, when you compare None (and to a lesser extent True and False) by value (==, !=) rather than by identity (is, is not) you can end up with unexpected results if the object you're comparing is buggy, etc. It's not a common problem, but when it happens it can be painful to track down, so it's better to have the habit of always using identity comparison rather than value comparison even if the object (in this case the regex matches) is (almost) certain to be bugfree.

The following should illustrate:

(Toggle Plain Text)

class Expected:
    pass

class Evil1:
    def __eq__(self, other):
        return self == other

class Evil2:
    def __cmp__(self, other):
        if other is self:
            return True
        return False

for cls in (Expected, Evil1, Evil2):
    x = cls()
    print cls.__name__
    print "   ", x == None, x != None, x is None, x is not None, "\n"

-T.
"One man's cargo cult is another man's defensive."

DaWei · May 23rd, 2006, 9:51 AM

Perhaps I oversimplify things. Rather than create examples, I reasoned thusly: Suppose I have two things that, compared as objects, yield one result. When compared by some attribute, they yield the opposite result. One of the two comparisons is "wrong." Which one is wrong depends on the requirements of the writer. The important thing, to me, is to be aware that there's a difference. I WAS NOT AWARE until the comment was made. I thank you for that and for your additional explanation. I will, I must admit, not ascribe evil or safety to one over the other, except as individual circumstance dictates.

I appreciate the help from both of you.

Dietrich · May 23rd, 2006, 12:06 PM

Assuming the code is correct, my measure of acceptable Python brevity is:

1) Let one of your peers look at it, and see if he/she can figure out how it works (in less than a day).

2) Imagine yourself looking at it after a few months and still know what it does.

Lastly, you can play innocent, put the code on the forum, and ask how it works. If nobody answers, you exceeded the brevity limit!

DaWei, may you be smitten with Python desire real good!

May 23rd, 2006, 9:51 AM	#12
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Perhaps I oversimplify things. Rather than create examples, I reasoned thusly: Suppose I have two things that, compared as objects, yield one result. When compared by some attribute, they yield the opposite result. One of the two comparisons is "wrong." Which one is wrong depends on the requirements of the writer. The important thing, to me, is to be aware that there's a difference. I WAS NOT AWARE until the comment was made. I thank you for that and for your additional explanation. I will, I must admit, not ascribe evil or safety to one over the other, except as individual circumstance dictates. I appreciate the help from both of you. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

May 23rd, 2006, 12:06 PM	#13
Dietrich Professional Programmer Join Date: Feb 2005 Posts: 434 Rep Power: 4	Assuming the code is correct, my measure of acceptable Python brevity is: 1) Let one of your peers look at it, and see if he/she can figure out how it works (in less than a day). 2) Imagine yourself looking at it after a few months and still know what it does. Lastly, you can play innocent, put the code on the forum, and ask how it works. If nobody answers, you exceeded the brevity limit! DaWei, may you be smitten with Python desire real good! __________________ I looked it up on the Intergnats! Last edited by Dietrich; May 23rd, 2006 at 12:16 PM.

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