Is there a standardized method to swap two variables in Python?

Question

In Python, I\'ve seen two variable values swapped using this syntax:

left, right = right, left

Is this considered the standard way to swap

Answer 1

You can combine tuple and XOR swaps: x, y = x ^ x ^ y, x ^ y ^ y

x, y = 10, 20

print('Before swapping: x = %s, y = %s '%(x,y))

x, y = x ^ x ^ y, x ^ y ^ y

print('After swapping: x = %s, y = %s '%(x,y))

or

x, y = 10, 20

print('Before swapping: x = %s, y = %s '%(x,y))

print('After swapping: x = %s, y = %s '%(x ^ x ^ y, x ^ y ^ y))

Using lambda:

x, y = 10, 20

print('Before swapping: x = %s, y = %s' % (x, y))

swapper = lambda x, y : ((x ^ x ^ y), (x ^ y ^ y))

print('After swapping: x = %s, y = %s ' % swapper(x, y))

Output:

Before swapping: x =  10 , y =  20
After swapping: x =  20 , y =  10

Answer 2

I know three ways to swap variables, but a, b = b, a is the simplest. There is

XOR (for integers)

x = x ^ y
y = y ^ x
x = x ^ y

Or concisely,

x ^= y
y ^= x
x ^= y

Temporary variable

w = x
x = y
y = w
del w

Tuple swap

x, y = y, x

Answer 3

To get around the problems explained by eyquem, you could use the copy module to return a tuple containing (reversed) copies of the values, via a function:

from copy import copy

def swapper(x, y):
  return (copy(y), copy(x))

Same function as a lambda:

swapper = lambda x, y: (copy(y), copy(x))

Then, assign those to the desired names, like this:

x, y = swapper(y, x)

NOTE: if you wanted to you could import/use deepcopy instead of copy.

Answer 4

Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side.

http://docs.python.org/3/reference/expressions.html#evaluation-order

That means the following for the expression a,b = b,a :

• the right-hand side b,a is evaluated, that is to say a tuple of two elements is created in the memory. The two element are the objects designated by the identifiers b and a, that were existing before the instruction is encoutered during an execution of program
• just after the creation of this tuple, no assignement of this tuple object have still been made, but it doesn't matter, Python internally knows where it is
• then, the left-hand side is evaluated, that is to say the tuple is assigned to the left-hand side
• as the left-hand side is composed of two identifiers, the tuple is unpacked in order that the first identifier a be assigned to the first element of the tuple (which is the object that was formely b before the swap because it had name b)
  and the second identifier b is assigned to the second element of the tuple (which is the object that was formerly a before the swap because its identifiers was a)

This mechanism has effectively swapped the objects assigned to the identifiers a and b

So, to answer your question: YES, it's the standard way to swap two identifiers on two objects.
By the way, the objects are not variables, they are objects.

Answer 5

I would not say it is a standard way to swap because it will cause some unexpected errors.

nums[i], nums[nums[i] - 1] = nums[nums[i] - 1], nums[i]

nums[i] will be modified first and then affect the second variable nums[nums[i] - 1].

Answer 6

Does not work for multidimensional arrays, because references are used here.

import numpy as np

# swaps
data = np.random.random(2)
print(data)
data[0], data[1] = data[1], data[0]
print(data)

# does not swap
data = np.random.random((2, 2))
print(data)
data[0], data[1] = data[1], data[0]
print(data)

See also Swap slices of Numpy arrays