CODEWITHSUNDEEP
python
Ante
Ante

Reputation: 251

Python a, b = b, a +b

This is my first question and I started to learn Python. Is there a difference between: 

a, b = b, a + b

and

a = b
b = a + b

When you write it in below example it shows different results.

def fib(n):
    a, b = 0, 1
    while a < n:
        print(a, end=' ')
        a, b = b, a + b
    print()
fib(1000)

and

def fib(n):
    a, b = 0, 1
    while a < n:
        print(a, end=' ')
        a = b
        b = a + b
    print()
fib(1000)

Upvotes: 24

Views: 60591

Answers (7)

Shaun T
Shaun T

Reputation: 1

There are differences between a,b = b,a+b and a=b b=a+b
let's have a look at the following two examples:

eg1:

a,b = 0,1
while a<10:
  print(a)
  a,b = b,a+b

#output:
0
1
1
2
3
5
8

eg2:

a,b = 0,1
while a<10:
  print(a)
  a=b
  b=a+b
#output:
0
1
2
4
8

This is because the interpreter always calculates the figures in the right side of the Equals sign first. The calculation results will be assigned to the variables which on the left hand side only if all the calculation has been done on the right hand side.

Upvotes: 0

Wizard
Wizard

Reputation: 22113

Let's grok it.

a, b = b, a + b

It's a tuple assignment, means (a, b) = (b, a + b), just like (a, b) = (b, a)

Start from a quick example:

a, b = 0, 1
#equivalent to
(a, b) = (0, 1)
#implement as
a = 0
b = 1

When comes to (a, b) = (b, a + b)
EAFP, have a try directly

a, b = 0, 1
a = b #a=b=1
b = a + b #b=1+1
#output
In [87]: a
Out[87]: 1
In [88]: b
Out[88]: 2

However,

In [93]: a, b = b, a+b
In [94]: a
Out[94]: 3
In [95]: b
Out[95]: 5

The result is different from the first try.

Tha's because Python firstly evaluates the right-hand a+b
So it equivalent to:

old_a = a
old_b = b
c = old_a + old_b
a = old_b
b = c

In summary, a, b = b, a+b means,
a exchanges to get old_value of b,
b exchanges to get the sum of old value a and old value b,

Upvotes: 1

Liwei
Liwei

Reputation: 481

I hope that you haven't been influenced by C language, which the priority of assignment operator = is higher than that of Comma operator ,. Do not think it's (a), (b = b), (a + b). It's a tuple assignment, meaning it's (a, b) = (b, a + b).

Upvotes: 0

SUMIT KHAJANCHI
SUMIT KHAJANCHI

Reputation: 1

a, b = b, a + b is similar to a, b = 0, 1 assigning values to both variables a, b at same time. First assign a = b and then b = a + b.

Upvotes: 0

Martijn Pieters
Martijn Pieters

Reputation: 1124000

The line:

a, b = b, a + b

is closer to:

temp_a = a
a = b
b = temp_a + b

where b is using the old value of a before a was reassigned to the value of b.

Python first evaluates the right-hand expression and stores the results on the stack, then takes those two values and assigns them to a and b. That means that a + b is calculated before a is changed.

See How does swapping of members in the python tuples (a,b)=(b,a) work internally? for the low-down on how this all works, at the bytecode level.

Upvotes: 20

Wooble
Wooble

Reputation: 90007

Let's say we start with a and b like this:

a = 2
b = 3

So, when you do:

a, b = b, a + b

what happens is you create the tuple (b, a + b) or (3, 5) and then unpack it into a and b so a becomes 3 and b becomes 5.

In your second example:

a = b
# a is now 3
b = a + b
# b is 3 + 3, or 6.

Upvotes: 13

isedev
isedev

Reputation: 19641

In a, b = b, a + b, the expressions on the right hand side are evaluated before being assigned to the left hand side. So it is equivalent to:

c = a + b
a = b
b = c

In the second example, the value of a has already been changed by the time b = a + b is run. Hence, the result is different.

Upvotes: 49

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