When two variables have the same identities, the is
returns a True, when they have different identities,
is returns a False,
even if their values are equal.
We use multiple assignment
to create two variables and assign them the same value.
Because both the variables hold the integer literal 257,
the expression p == q results in a True
as per the functioning of
comparison operators.
In the expression p is q,
the identities of the two variables are compared.
Let us check their identities:
We see that the identities of
p and q are the same.
So p is q returned True.
The above results conclude that both the variables contain the same literal 257,
and they point to the same integer object.
However, there is an exception. If we define the variables seperately,
as done in the following, the output differs:
Here, two different objects with different identities
are created, so p is q
returned False.
This exception occurs only in interactive mode.
In script mode, with the above code p is q
will return True.
The following statements when executed in interactive mode
show that the is operator
behaves unexpectedly with different literals.
Code
p is q
Objects created
Identities
p = -6
False
two
different
q = -6
p = -5
True
one
same
q = -5
p = 256
True
one
same
q = 256
p = 257
False
two
different
q = 257
In python, small integers in the range of -5 to 256
are treated differently than the other integers.
However, to avoid any confusion, it is recommended not to use the
is operator which compares identities,
rather use the == operator which compares values.
The above recommendation specially applies for immutable objects like 'int', 'float', 'str' etc.
The is not operator
The is not operator is exactly
opposite to the is operator.
If x is y returns False,
x is not y
will return True.
x is y returns False,
so x is not y returns True.
n is m returns True,
so n is not m returns a False.
Since n is m and n is not m are valid
python expressions, they can be assigned to variables.
Python membership operators in and not in
Before going into the details of python identity and membership operators,
please go through the page
python lists and
python tuples
to understand the conception of python collection objects.
Python has two membership operators
to check whether a certain object is present in a collection of objects.
Operator
Syntax
in
value in collection
not in
value not in collection
Python membership operator in
The operator in checks whether
an item is a member of a collection object like list, tuple etc.
If the item is member, a True is returned,
else a False is returned.
We create a variable mylist and assign a list to it.
In the above, is the item 1
is a member of the list [1, 3, 5, 7, 9] ? Yes, true.
The first element of the list is 1.
So, a True is returned.
Is the item 10 is a member of the list
? No, false. 10 does not belong to this list.
So, a False is returned.
We can even do this without any variable as in the following:
Note that numeric literal 7
is present in the list as the 3rd element,
so True is the output.
But no string literal "7" is present in the given list,
so a False is returned in response to the fourth line.
We have already seen that "7"
and 7 are not the same in python.
Python tuples are also immutable collection of objects. With the
in and not in
we can check if a certain data is present in a tuple.
Both "Assam" and
False values are members of the given tuple.
So both expressions return True.
Neither 2718 nor 85
are present in the given tuple. So both expressions return False.
Note again that 85 and "85" are not the same in python.
In the above, 10 in mylist,
85 in mytuple, 7 in [1, 3, 5, 7, 9]
are all valid boolean
expressions.
They evaluate to either True or False values.
Python strings are sequence of characters, so with membership operators
we can check if a particular character is present in a given string or not.
We can even check if a particular word is part of the string or not:
Python membership operator not in
The operator not in
is the inverse of in operator.
With not in operator, if the operand
is a member of the collection, a False is returned,
else a True is returned.
Because 1 in mylist
results a True,
∴ 1 not in mylist
will result a False.
Because 10 in mylist
results a False,
∴ 10 not in mylist
will result a True.
The character "Z" is not
a part of the string "INDIA",
so "Z" not in "INDIA"
returns a True.
The character "D" is
a part of the string "INDIA",
so "D" not in "INDIA"
returns a False.
Two lists and a number are given. Write a boolean expression
which is evaluated to True
if the given number is present in both of the lists,
else the expression shoud return a False.
We write a script to implement the above.
The above logic is written correctly. In the fourth line, if you write
the logic as number in evens and primes,
you will get an unexpected result.
Now change the value of number
to anything except 2,
you will get the output False.
