Python: Usage of Variables and their difference (“a, b = 0, 1” VS “a = 0”, “b = 1”) [duplicate]

问题

I was looking at the Python Manual and found this snippet for a Fibonacci-Number generator:

def fib(n):    # write Fibonacci series up to n
    a, b = 0, 1
    while b < n:
        print(b, end=' ')
        a, b = b, a+b
    print()

The output is dependent on n and returns a valid Fibonacci sequence.

If you remodel this to use the variables "a" and "b" seperately like so:

def fib(n):    # write Fibonacci series up to n
    a = 0
    b = 1
    while b < n:
        print(b, end=' ')
        a = b
        b = a+b
    print()

then it will print a number sequence that increments by the power of 2 (e.g. 1, 2, 4, 8, 16 and so on).

So I was wondering why that happens? What is the actual difference between the two uses of variables?

回答1:

Doing:

a, b = b, a+b

is equivalent to:

temp = a
a = b
b += temp

It lets you simultaneously do two calculations without the need of an intermediate/temporary variable.

The difference is that in your second piece of code, when you do the second line b = a+b, you have already modifed a in the previous line which is not the same as the first piece of code.

Examples

>>> a = 2
>>> b = 3
>>> a,b
2 3
>>> a,b = b,a
>>> a,b
3 2

On the other hand, if you use the second approach shown in your question:

>>> a = 2
>>> b = 3
>>> a,b
2 3
>>> a = b
>>> b = a
>>> a,b
3 3

回答2:

    a, b = b, a+b

the right-hand expressions are evaluated first, and their results are assigned to a and b. This is similar to the following:

    _new_a = b
    _new_b = a+b
    a = _new_a
    b = _new_b

On the other hand, in

    a = b
    b = a+b

you are modifying a before adding it to b. This is equivalent to

    a, b = b, b+b

which explains where the powers of two are coming from.

来源：https://stackoverflow.com/questions/23085008/python-usage-of-variables-and-their-difference-a-b-0-1-vs-a-0-b

标签

python

variables

fibonacci