Python generator behaviour

Question

import itertools
ws=[]
subs=[]
set_subs=[]
for i in xrange(int(raw_input())):
    S=raw_input()
    l=len(S)
    subs.append(S[i:j+1] for i in xrange(l) for j in xrange(i,l))

input:

2
aab
aac

now both subs[0] and subs[1] give me same result.

print list(subs[0])
>>>['a','aa','aac','a','ac','c']
print list(subs[1])
>>>['a','aa','aac','a','ac','c']

whereas list(subs[0]) should have been ['a','aa','aab','a','ab','b']

I vaguely understand why this is happening. What do I do to make subs[0] and subs[1] actually different.

NOTE: changing the line

subs.append(S[i:j+1] for i in xrange(l) for j in xrange(i,l))

with

subs.append([S[i:j+1] for i in xrange(l) for j in xrange(i,l)])

is not an option

just an advise: you shouldn't reuse i as variable in your generator expression. It can lead to confusion. — mata
– mata, Commented May 24, 2012 at 8:43

Janne Karila · Accepted Answer · 2012-05-24 08:41:25Z

http://docs.python.org/reference/expressions.html#generator-expressions

Variables used in the generator expression are evaluated lazily when the __next__() method is called for generator object (in the same fashion as normal generators). However, the leftmost for clause is immediately evaluated, so that an error produced by it can be seen before any other possible error in the code that handles the generator expression. Subsequent for clauses cannot be evaluated immediately since they may depend on the previous for loop.

S[i:j+1] is evaluated when you execute the generator, and at that point S has the latest value.

You can use a normal generator instead. Now ss is local to subgen:

import itertools

def subgen(ss):
    l=len(ss)
    for i in xrange(l):
        for j in xrange(i,l):
            yield ss[i:j+1]

subs=[]
for i in xrange(int(raw_input())):
    S=raw_input()
    subs.append(subgen(S))

Mariy · Accepted Answer · 2012-05-24 08:36:29Z

2

Well, this is one of the strange features of generator expression. Have a look at this In your case it is used late binding and that's why you will get two identical results.

answered May 24, 2012 at 8:36

Mariy

5,9644 gold badges44 silver badges60 bronze badges

Collectives™ on Stack Overflow

Python generator behaviour

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related