问题
When in a Python interactive session:
In [1]: a = "my string"
In [2]: b = "my string"
In [3]: a == b
Out[3]: True
In [4]: a is b
Out[4]: False
In [5]: import sys
In [6]: print(sys.version)
3.5.2 (default, Nov 17 2016, 17:05:23)
[GCC 5.4.0 20160609]
On the other hand, when running the following program:
#!/usr/bin/env python
import sys
def test():
a = "my string"
b = "my string"
print(a == b)
print(a is b)
if __name__ == "__main__":
test()
print(sys.version)
The output is:
True
True
3.5.2 (default, Nov 17 2016, 17:05:23)
[GCC 5.4.0 20160609]
Why a is b has different outcome in the above two cases?
I am aware of this answer (and of course the difference between the == and is operators! that is the point of the question!) but aren't a and b the same object also in the first case? (interpeter?) since they point to the same (immutable) string?
回答1:
This is caused by string interning. See this question for another example.
In your example, CPython interns the string constants in the module but doesn't in the REPL.
回答2:
So the console creates two different objects when creating two strings, but the interpreter, when running code in a single function will reuse the memory location of identical strings. Here is how to check if this is happening to you:
a = "my string"
b = "my string"
print id(a)
print id(b)
If these two ids are the same, then a is b will return True, if not then it will return False
Looks like you are using anaconda, so I checked this in the console and found different ids and then wrote a function in the editor and executed it and got the same ids.
Note: Now that we know that is determines if two variable labels point to the same object in memory, I should say that is should be used sparingly. It is usually used to compare singletons like None a is None, for example. So don't use it to compare objects, use ==, and when creating classes implement the __eq__ method so you can use the == operator.
来源：https://stackoverflow.com/questions/41839886/python-not-interning-strings-when-in-interactive-mode