python: string comparison do not work as expected

Question

My program is like this:

filename=sys.argv[1]
print "filename is default?", (filename is "default")
if (filename is "default"):
  filename="..."
readfile(filename)

I type python ....py default in the command line. Then the output is:

filename is default? False

IOError:...No such file or directory 'default'.

I use pdb, and before the if statement excutes, p filename returns: 'default'.

see http://stackoverflow.com/questions/2988017/string-comparison-in-python-is-vs — reynoldsnlp, Dec 01 '15 at 09:17

tombam95 · Answer 1 · 2015-12-01T09:27:34.030

3

This is what you're looking for:

if filename == "default" :

The == operator is used for comparison, whilst the is operator tests if two variables point to the same object, not if two variables have the same value.

edited Dec 01 '15 at 09:27

answered Dec 01 '15 at 09:16

tombam95

343
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score 3 · Answer 2 · answered Dec 01 '15 at 09:16

3

Use == two compare whether two strings are equal.

Use is to test whether it is the same string.

answered Dec 01 '15 at 09:16

zvone

18,045
3
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77

BTW, I doubt you will *ever* want to compare string identities using `is`. – zvone Dec 01 '15 at 09:17
Hmm, if I try `a = "foo"; b = "foo"; print (a is b), (a == b)` I get two times `True`, any ideas why? – adrianus Dec 01 '15 at 09:27
@adrianus to the interpreter, `"foo"` is a constant. Check out my answer for a more detailed explanation. – Daniel Hepper Dec 01 '15 at 10:04
@DanielHepper Aah, cool, thanks! I also just noticed that `a = "foo"; b = "foo"; print id(a), id(b)` returns the same value two times... – adrianus Dec 01 '15 at 10:08
@adrianus That is an optimization trick. Interpreter is using the same variable for a and b because it "knows" that they are the same. On the other hand, when you say `filename=sys.argv[1]`, interpreter does not optimize that, because it does not know beforehand what `sys.argv[1]` would be. – zvone Dec 01 '15 at 14:09

score 1 · Accepted Answer · answered Dec 01 '15 at 10:02

Short answer:

if filename == "default" :

Long answer:

is checks for object identity. To check for equality, use ==. Check the Python documentation on comparisons. In your case:

Note that comparing two string constants with is will actually return true.

def f():
    a = "foo"
    b = "foo"
    print(a is b)   # True, because a and b refer to the same constant
    x = "f" + "oo"
    print(a is x)   # True, because the addition is optimized away
    y = "f"
    z = y + "oo"    #
    print(a is z)   # False, because z is actually a different object

You can see what happens under the hood by disassembling the CPython byte code:

>>> import dis
>>> dis.dis(f)
2         0 LOAD_CONST               1 ('foo')
          3 STORE_FAST               0 (a)

3         6 LOAD_CONST               1 ('foo')
          9 STORE_FAST               1 (b)

4        ...

5        28 LOAD_CONST               4 ('foo')
         31 STORE_FAST               2 (x)

6        ...

7        50 LOAD_CONST               2 ('f')
         53 STORE_FAST               3 (y)

8        56 LOAD_FAST                3 (y)
         59 LOAD_CONST               3 ('oo')
         62 BINARY_ADD
         63 STORE_FAST               4 (z)

9        ...

python: string comparison do not work as expected

3 Answers3