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Aug 14th, 2009

How does python evaluate conditional statements?

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(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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nunos

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Aug 14th, 2009

Re: How does python evaluate conditional statements?

Click to Expand / Collapse Quote 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:

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
>>>

jlm699

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Aug 14th, 2009

Re: How does python evaluate conditional statements?

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

Here's the code:

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:

(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:

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

Even though this last piece of code works I still would like to know why the error occurs.

Thanks.

nunos

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Re: How does python evaluate conditional statements?

The logical and in python is the and operator, not the & which is bitwise and.

Gribouillis

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Aug 14th, 2009

Re: How does python evaluate conditional statements?

Click to Expand / Collapse Quote originally posted by Gribouillis ...

The logical and in python is the and operator, not the & which is bitwise and.

Thanks. I changed '&' to 'and' and it now works as it should.

Cheers.

Last edited by nunos; Aug 14th, 2009 at 7:32 pm.

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