Confused about `is` operator with strings

Question

The is operator compares the memory addresses of two objects, and returns True if they're the same. Why, then, does it not work reliably with strings? Code #1

>>> a = "poi"
>>> b = "poi"
>>> a is b
True

Code #2

>>> ktr = "today is a fine day"
>>> ptr = "today is a fine day"
>>> ktr is ptr
False

I have created two strings whose content is the same but they are living on different memory addresses. Why is the output of the is operator not consistent?

Answer 1

I believe it has to do with string interning. In essence, the idea is to store only a single copy of each distinct string, to increase performance on some operations.

Basically, the reason why a is b works is because (as you may have guessed) there is a single immutable string that is referenced by Python in both cases. When a string is large (and some other factors that I don't understand, most likely), this isn't done, which is why your second example returns False.

EDIT: And in fact, the odd behavior seems to be a side-effect of the interactive environment. If you take your same code and place it into a Python script, both a is b and ktr is ptr return True.

a="poi"
b="poi"
print a is b  # Prints 'True'

ktr = "today is a fine day"
ptr = "today is a fine day"
print ktr is ptr  # Prints 'True'

This makes sense, since it'd be easy for Python to parse a source file and look for duplicate string literals within it. If you create the strings dynamically, then it behaves differently even in a script.

a="p" + "oi"
b="po" + "i"
print a is b  # Oddly enough, prints 'True'

ktr = "today is" + " a fine day"
ptr = "today is a f" + "ine day"
print ktr is ptr  # Prints 'False'

As for why a is b still results in True, perhaps the allocated string is small enough to warrant a quick search through the interned collection, whereas the other one is not?

Answer 2

is is identity testing. It will work on smaller some strings(because of cache) but not on bigger other strings. Since str is NOT a ptr. [thanks erykson]

See this code:

>>> import dis
>>> def fun():
...   str = 'today is a fine day'
...   ptr = 'today is a fine day'
...   return (str is ptr)
...
>>> dis.dis(fun)
  2           0 LOAD_CONST               1 ('today is a fine day')
              3 STORE_FAST               0 (str)

  3           6 LOAD_CONST               1 ('today is a fine day')
              9 STORE_FAST               1 (ptr)

  4          12 LOAD_FAST                0 (str)
             15 LOAD_FAST                1 (ptr)
             18 COMPARE_OP               8 (is)
             21 RETURN_VALUE

>>> id(str)
26652288
>>> id(ptr)
27604736
#hence this comparison returns false: ptr is str

Notice the IDs of str and ptr are different.

BUT:

>>> x = "poi"
>>> y = "poi"
>>> id(x)
26650592
>>> id(y)
26650592
#hence this comparison returns true : x is y

IDs of x and y are the same. Hence is operator works on "ids" and not on "equalities"

See the below link for a discussion on when and why python will allocate a different memory location for identical strings(read the question as well).

When does python allocate new memory for identical strings

Also sys.intern on python3.x and intern on python2.x should help you allocate the strings in the same memory location, regardless of the size of the string.