[Tutor] if >= 0

[Peter Otten]

rahmad akbar wrote:

> hey again guys, i am trying to understand these statements,
> 
> if i do it like this, it will only print the 'print locus' part
> 
> for element in in_file:
>   if element.find('LOCUS'):
>     locus += element
>   elif element.find('ACCESSION'):
>     accession += element
>   elif element.find('ORGANISM'):
>     organism += element
> 
> print locus
> print accession
> print organism
> 
> 
> once i add >= 0 to each if conditions like so, the entire loop works and
> prints the 3 items
> 
> for element in in_file:
>   if element.find('LOCUS') >= 0:
>     locus += element
>   elif element.find('ACCESSION') >= 0:
>     accession += element
>   elif element.find('ORGANISM') >= 0:
>     organism += element
> 
> print locus
> print accession
> print organism
> 
> why without '>= 0' the loop doesnt works?

element.find(some_string) returns the position of some_string in element, or 
-1 if some_string doesn't occur in element. All integers but 0 are true in a 
boolean context, i. e.

if -1: print -1 # printed
if 0: print 0 # NOT printed
if 123456789: print 123456789 # printed

So unlike what you might expect

if element.find(some_string): # WRONG!
    print "found"
else:
    print "not found"

will print "not found" if element starts with some_string and "found" for 
everything else.

As you are not interested in the actual position of the potential substring 
you should rewrite your code to

if "LOCUS" in element:
    locus += element
elif "ACCESSION" in element:
    accession += element
elif "ORGANISM" in element:
    organism += element

which unlike the str.find() method works as expected.