Comments on: Python common mistakes

By: diegoe

diegoe — Thu, 28 May 2009 20:16:15 +0000

@apol: Mmm, good idea. Will remember that next time, thanks.

By: apol

apol — Wed, 27 May 2009 17:44:04 +0000

When showing code mistakes, never mark the wrong ones in bold but the correct ones. You will make your point better.

By: engla

engla — Mon, 25 May 2009 22:21:46 +0000

@stephan: It is pythonic if the condition is long and in several parts (like .. && .. || .. ) etc. The pythonic way to do line continuations is using brackets.

By: Vincent Noel

Vincent Noel — Mon, 25 May 2009 12:26:07 +0000

Instead of “if a is not None” and “if a is None”, can’t we just write “if a” and “if not a”? This solution is advocated here: http://jaynes.colorado.edu/PythonIdioms.html

By: Stephan Puchegger

Stephan Puchegger — Mon, 25 May 2009 07:44:27 +0000

I agree that

if (condition):

is not “pythonic” and does not adhere the PEP 8 style guidelines, but it is not a mistake, since it is valid and does what one would expect it to do. Yes the “(” and “)” are unneeded in this case, but for some they are a welcome sign where the condition begins and where it ends. A style guideline is not a law after all and those who wrote the style guideline introduced their own subjective preferences.

By: Paddy3118

Paddy3118 — Mon, 25 May 2009 04:20:39 +0000

@rvl:
Your item 3 on Duck Typing, is it a problem for you? I find Duck Typing works fine for me.

By: diegoe

diegoe — Sun, 24 May 2009 18:16:19 +0000

@Sebastian nice explanation, thank you

By: diegoe

diegoe — Sun, 24 May 2009 18:14:19 +0000

@mezo: mmm, didn’t get that one

By: mezo

mezo — Sun, 24 May 2009 18:09:06 +0000

I see often something like this:

a = ['kasdoasudiasud90as9u8d0asdijasodjaiso', 'ksdaosjdiasu9023u8e9i0ajdi23ui0euqw0du9q0','0-230asd0a-s9d0-as9d0-asd0a-sdi0-asid0as-di0as-sdia-']
for x in a:
paint(thisandthat)
instead of for int in a.

By: Sebastian

Sebastian — Sun, 24 May 2009 10:24:19 +0000

The difference between “==” and “is” is that the “==” operator calls the __eq__() or __cmp__() method on the first object, and “is” doesn’t. It is possible for some instance or other to compare positively to None, even though that behaviour would be surprising and should be avoided.

Example:

class AlwaysEqual:

def __eq__(self, other):
print ‘__eq__() called’
return True

x = AlwaysEqual()

print x == None
print x is None

Since someone who compares a variable to None will usually want to know whether or not it has been instantiated, he should use the “is” operator.

This is made rather more explicit in some other languages such as Java, where you have to call one of the two object’s equals() method explicitly. The world’s log files are full of NullPointerExceptions where someone called equals() on null. This is why a general coding recommendation is to always write

“hello”.equals(greeting)

instead of

greeting.equals(”hello”)