list + = str和list + = str有什么区别，

Question

Today I noticed a strange behavior while doing some python list operations.

Lets say,

a = []
b = 'xy'

When I do, a += b the interpreter returns:

a += b
a == ['x', 'y']

but when I do, a += b, (with a comma) the interpreter returns a = ['xy']

a += b,
a == ['xy']

Can someone please explain what is happening here.

Answer 1

a += b

When a is a list, this operation is similar to a.extend(b) . So it iterates the object b , appending each element to a .

If you iterate the string 'xy' , it yields two elements 'x' and 'y' .

If you iterate the tuple 'xy', , it yields one element 'xy' .

Answer 2

The line

a += b,

is equivalent to

a += (b, )

It creates a tuple with one item. If it is added, the item 'xy' it added to a .

If you add a string like 'xy' it counts as a sequence of characters on it's own and every sequence item (character) is added individually to a .

So basically the comma wraps b into a tuple.

Answer 3

Comma creates tuple - so a += b, means the same as a += (b,)

a += ('xy', )

You add tuple with one element so it almost like

a += ['xy']

so you have

a = [] + ['xy']

which gives result

a = ['xy']