Understanding Python Bitwise, Arithmetic, And Boolean Operators

I'm new to Python and not able to understand this. Can someone help break down the statement for me? Both n and parity are integers n += parity != n & 1

Solution 1:

The expression is evaluated as n += (parity != (n & 1)), and the results are:

• n & 1 is a bitmask, it masks the integer n down to the least-significant bit. If n is odd, it will be 1, if it is even, that bit will be 0.

• parity != 0 or parity != 1 produces a boolean result, True or False, signalling if parity was not equal to the 0 or 1 on the right.

• The resulting True or False is added up to n, as if you did n = n + True or n = n + False. The Python boolean type is a subclass of int and False has an integer value of 0 and True a value of 1.

The code, in essence, is adding 0 or 1 to n based on the value of parity and if n is currently even or odd.

A short demo may illustrate this better.

First, n & 1 producing 0 or 1:

>>>n = 10# even>>>bin(n)  # the binary representation of 10
'0b1010'
>>>n & 1# should be 0, as the last bit is 0
0
>>>n = 11# odd>>>bin(n)  # the binary representation of 11
'0b1011'
>>>n & 1# should be 1, as the last bit is 1
1

Next, the parity != 0 or parity != 1 part; note that I assume parity is limited to 0 or 1, it really doesn't make sense for it to have other values:

>>>parity = 0>>>parity != 1
True
>>>parity != 0
False
>>>parity = 1>>>parity != 1
False
>>>parity != 0
True

Last, that booleans are integers:

>>>isinstance(True, int)
True
>>>int(True)
1
>>>10 + True
11
>>>10 + False
10

The formula looks like it is calculating a CRC checksum.

Solution 2:

Lets break this down:

(n += (parity != (n & 1)))

(n & 1) this is bitmask, and takes the value of the smallest (least significant bit) of n.

parity != this is true if parity is different from the result of (n & 1)

n += this increments n by whatever value the rest of the line returns.

n    parity    output(increment of n)
011110101000

From the above table you can see that it functions like an XOR of n's LSB and parity.

Notes: Usually parity is the oddness(1) or evenness(0) of a data packet.

Hope this helps! Enjoy.