The = operator is essential to
create a new python variable, and used frequently in programming.
In the above two variables x and y
have been defined.
A numeric literal
has been assigned to x, and
an expression
is assigned to y.
Both are assignment statements which do not produce
any output, but multiple actions have been taken internally.
When the statement x = 257 is executed,
an integer object 257 is created
a variable x is created and memory address of the object is stored in it
data-type of the variable is confirmed as 'int'
The data-type of the variables will be reflected in the output of type().
We can now technically say that x points to the integer object 257,
or x refers to the integer object 257.
When the statement y = x + 3.5 is executed,
the expression x + 3.5 is evaluted to 260.5
a float object 260.5 is created
its memory address is stored in variable y
data-type of the variable is confirmed as 'float'
In an assignment statement, the left hand side must be variable,
and the right hand side must be a valid python expression.
The statement aaa = bbb + 5 * xxx
is completely valid, but 1 = x is an invalid statement.
Let us find some other forms of assigning variables.
Parallel assignment
In this format, multiple variable names are kept to the
left side of the = operator,
while multiple values are kept to the right side of the operator.
In line 2, variables a and b are assigned
the values 100 and
200 respectively. In line 5,
a string literal "pen" is assigned to variable item,
a float literal 24.99 is assigned to price and
an integer literal 300 is assigned to variable quantity.
When two variables interchange their values, we call it swapping of variables. In the above,
value of a goes to b, while
value of b goes to a.
Multiple assignment
Variables x, y and
z get the same integer literal
42. All of them point to the same object.
Variables counter1 and
counter2 are initialized to zero.
Both of them point to the same object.
Assignment by equalising
Variables x and
y contain the same integer literal
-6. Both of them point to the same object.
Augmented assignment operators
Another type of assignment is frequently found in programming languages:
i = i + 1
This statement can never be true mathematically,
but it is an important syntax in programming,
where the value of variable i is increased by 1.
The variable i must be defined prior to the execution of the statement.
The fourth statement counter = counter + 1 interprets like the following: add 1 to the existing value of couner,
and store the result incouner.
Augmented assignment operator +=
Now, instead of i = i + 1, the operator += can be used:
Executing counter += 1 is the same as
executing counter = counter + 1(there is an exception though).
Another example goes like this:
For every augmented assignment statements, the variable involved must be defined earlier
(before the execution of the statement).
Augmented assignment operator -=
In the 2nd assignment statement a -= 5,
variable a holds the literal -3
(a = a - 5=>a = 2 - 5=>a = -3).
Augmented assignment operator *= and /=
Instead of b = b * 10,
assignment statement b *= 10 is used.
Instead of myvar = myvar / 2,
assignment statement myvar /= 2 is used.
Augmented assignment operator **=
Instead of number = number ** 4,
assignment statement number **= 4 is used which is much shorter.
Augmented assignment operator //=
Instead of x = x // 5,
assignment statement x //= 5 is used.
Augmented assignment operator %=
Instead of y = y % 7,
assignment statement y %= 7 is used.
The walrus operator :=
We have used many assignment statements in our earlier examples.
These statements do not produce any output. In other words, they do not return anything.
The interpreter silently assigns the integer literal 99 to the variable number,
and waits for next input in the next line.
To display the assigned value we need a second input:
As IDLE window allows to omit the print,
We have just entered the variable name in the second line, and the output is displayed.
The above assignment in the first line is a simple assignment statement.
Variable assignment with the walrus operator takes the following form:
(variable:=literal or expression)
The walrus operator := assigns
the literal 99 to variable number, as well as
displays the value of the variable.
Unlike simple assignment statements, the walrus operator produces an output, or
in a programmers's words, it returns a value.
Such a statement, containing the python walrus operator, is known as an assignment expression.
It performs two tasks: assigns a value to a variable, and returns the value too.
A pair of parentheses is mandatory in such expressions, without which, the interpreter will raise an error .
To implement the utility of the walrus operator :=,
we reqiure python conditional branching if else
or python loops using for or while.
Since we are yet to learn how to use loops in python, let us try the following example.
Suppose we have to display four consecutive odd numbers by incrementing
a variable next_odd, given the first one:
Now we utilize the walrus operator := to get the same output.
Of course we shall find better utility of the walrus operator
while doing conditional branching and loops in python.
Next, we shall delve into operator precedence in python.
