when do two names cease to refer to the same string object?

[John Salerno]

To test this out a wrote a little script as an exercise:

for num in range(1, 10):
x = 'c' * num
y = 'c' * num

if x is y:
print 'x and y are the same object with', num, 'characters'
else:
print 'x and y are not the same object at', num, 'characters'
break

But a few questions arise:

1. As it is above, I get the 'not the same object' message at 2
characters. But doesn't Python only create one instance of small strings
and use them in multiple references? Why would a two character string
not pass the if test?

2. If I say x = y = 'c' * num instead of the above, the if test always
returns true. Does that mean that when you do a compound assignment like
that, it's not just setting each variable to the same value, but setting
them to each other?

Finally, I'd like to see how others might write a script to do this
exercise. Basically I just wanted to see when two names stop referring
to the same string, which I figured was based on length of the string.
But I've also noticed that putting spaces in the string also affects
things, so maybe there isn't just one way to test this.

 

[Farshid Lashkari]

I asked a similar question a few weeks ago. I think the answers I got
will also answer your question as well. Here's a link to the thread:

http://groups.google.com/group/comp...ffe44149?lnk=st&rnum=1&hl=en#f564a219ffe44149

-Farshid

 

[John Salerno]

I asked a similar question a few weeks ago. I think the answers I got
will also answer your question as well. Here's a link to the thread:

http://groups.google.com/group/comp...ffe44149?lnk=st&rnum=1&hl=en#f564a219ffe44149

-Farshid

Thanks! Very interesting stuff. Seems like basically that sometimes
strings are reused, and sometimes they are created anew. I also
understand the point about not relying on the implementation, but like
you I was just curious.

 

[Steven D'Aprano]

To test this out a wrote a little script as an exercise:

for num in range(1, 10):
x = 'c' * num
y = 'c' * num

if x is y:
print 'x and y are the same object with', num, 'characters'
else:
print 'x and y are not the same object at', num, 'characters'
break

But a few questions arise:

1. As it is above, I get the 'not the same object' message at 2
characters. But doesn't Python only create one instance of small strings
and use them in multiple references? Why would a two character string
not pass the if test?

Watch this:
False


Does this give you a hint as to what is happening?

Some more evidence:

"aaaaa"[0:1] is "aaaaa"[0:1] True
"aaaaa"[0:2] is "aaaaa"[0:2]

False


2. If I say x = y = 'c' * num instead of the above, the if test always
returns true. Does that mean that when you do a compound assignment like
that, it's not just setting each variable to the same value, but setting
them to each other?

Yes. Both x and y will be bound to the same object, not just two objects
with the same value. This is not an optimization for strings, it is a
design decision for all objects:

x = y = []
x.append(1)
y

[1]


Finally, I'd like to see how others might write a script to do this
exercise.

filename = "string_optimization_tester.py"
s = "if '%s' is not '%s':\n raise ValueError('stopped at n=%d')\n"
f = file(filename, "w")
for n in range(1000):
f.write(s % ("c"*n, "c"*n, n))

f.write("""if 'ccc' is not 'c'*3:
print 'Expected failure failed correctly'
else:
print 'Expected failure did not happen'
""")
f.write("print 'Done!'\n")
f.close()
execfile(filename)