Aren't Python strings immutable? Then why does a + " " + b work?
Solution 1:
First a
pointed to the string "Dog". Then you changed the variable a
to point at a new string "Dog eats treats". You didn't actually mutate the string "Dog". Strings are immutable, variables can point at whatever they want.
Solution 2:
The string objects themselves are immutable.
The variable, a
, which points to the string, is mutable.
Consider:
a = "Foo"
# a now points to "Foo"
b = a
# b points to the same "Foo" that a points to
a = a + a
# a points to the new string "FooFoo", but b still points to the old "Foo"
print a
print b
# Outputs:
# FooFoo
# Foo
# Observe that b hasn't changed, even though a has.
Solution 3:
The variable a is pointing at the object "Dog". It's best to think of the variable in Python as a tag. You can move the tag to different objects which is what you did when you changed a = "dog"
to a = "dog eats treats"
.
However, immutability refers to the object, not the tag.
If you tried a[1] = 'z'
to make "dog"
into "dzg"
, you would get the error:
TypeError: 'str' object does not support item assignment"
because strings don't support item assignment, thus they are immutable.