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How does python evaluate conditional statements?

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Join Date: Aug 2009

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nunos

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How does python evaluate conditional statements?

Aug 14th, 2009

(a | b):
For what I have read in other forum post, if a is true, python doesn't evaluate b an returns a.

(a & b):
What happens with logical and? If a is false, does python evaluates b or not?

Thanks for your help.

Last edited by nunos; Aug 14th, 2009 at 4:30 pm.

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Re: How does python evaluate conditional statements?

Aug 14th, 2009

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Originally Posted by nunos

(a & b):
What happens with logical and? If a is false, does python evaluates b or not?

No. Here's the code I just used to check this:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
>>> def a():
...     print 'a'
...     return False
...     
>>> def b():
...     print 'b'
...     return True
...     
>>> a() and b()
a
False
>>> b() and a()
b
a
False
>>>
>>> def a():
...     print 'a'
...     return False
...     
>>> def b():
...     print 'b'
...     return True
...     
>>> a() and b()
a
False
>>> b() and a()
b
a
False
>>>

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Re: How does python evaluate conditional statements?

Aug 14th, 2009

The reason I asked about the evaluation of the logical and was because of this error I keep getting.

Here's the code:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
import re
 
m = "03-AA-12"
 
x = re.compile("[A-Z]{2,2}-[0-9]{2,2}-[0-9]{2,2}")
 
objx = x.match(m)
 
if (objx != None) & (objx.group() == m):
    print "yes"
else:
    print "no"
import re

m = "03-AA-12"

x = re.compile("[A-Z]{2,2}-[0-9]{2,2}-[0-9]{2,2}")

objx = x.match(m)

if (objx != None) & (objx.group() == m):
    print "yes"
else:
    print "no"

Here's the error:

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(Toggle Plain Text)

Traceback (most recent call last):
  File "D:\Workspace\py.LAB\src\lab.py", line 9, in <module>
    if (objx != None) & (objx.group() == m):
AttributeError: 'NoneType' object has no attribute 'group'

According to what was mentioned before, I can't understand why this error occurs.

Python should only evaluate objx.group() == m if objx != None is True, since it is False it should not evaluate it and this error would not occur.

To overcome this I nested two if statements, like so:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
if objx != None:
    if objx.group() == m:
        print "yes"
if objx != None:
    if objx.group() == m:
        print "yes"