Variables in Python

Variable : It is a symbolic name that is a reference or pointer to an object. Once an object is assigned to a variable, you can refer to the object by that name. Example:
a=5
b=10.78
c=2 + 7j
d=”Gargs Academy”
e=True
f=None
a,b,c,d,e,f are variables of different types. We can assign any type of value to a variable in python.

left value or l-value or locator value: It is an assignable object. It is any expression that may occur on the leftside of an assignment. Variables names are l-values. Example:
a=5
b=10.78
c=2 + 7j
d=”Gargs Academy”
e=True
f=None
a,b,c,d,e,f are l-values

right value or r-value: It is any expression that has a value that may appear on the right of an assignment. Example:
a=5
b=10.78
c=2 + 7j
d=”Gargs Academy”
e=True
f=None
5, 10.78, 2+7j, “Gargs Academy”, True, None are r-values

Types of Values in Python: Following types of values are supported in python.
1. Numeric (Integer, Float, Complex)
2. Boolean
3. Sequences/ Collections (List, Tuple, Set, Strings)
4. Mapping ( Dictionary)
5. Special Data Type None

1. Numeric Values: These are values in the form of numbers. Python supports Integer, Float and Complex values.

a) Integer Value It is any combination of digits along with +/- sign. It includes both positive and negative numbers along with 0. It doesn’t include fractional parts.

>>> a=4398
>>> a
4398
>>> a=-34534
>>> a
-34534
>>> a=0
>>> a
0

b) Float Values: It includes both positive and negative real numbers. It also includes fractional parts. There are two forms to represent float literals: I) standard form and II) mantissa exponent form.

I) Standard Form: Any number with or without +/- sign having decimal point

>>> a=54.645
>>> a
54.645
>>> a=-324.654
>>> a
-324.654
>>> a=9.
>>> a
9.0
>>> a=.7
>>> a
0.7

II) Mantissa Exponent Form: mantissa may have any number with or without +/- sign having decimal point, and exponent must be a positive or negative integer followed by e/E sign. Internally python stores the number in normalized form and using e sign for exponent.

>>> a=44.3432e34
>>> a
4.43432e+35
>>> a=0.00324E-45
>>> a
3.24e-48
>>> a=0.00324E-4.5
SyntaxError: invalid syntax

There must not be any space between mantissa, e and exponent part.

>>> a=44.3432 e 34
SyntaxError: invalid syntax
>>> a=44.3432 e34
SyntaxError: invalid syntax

c) Complex Values: It is of the form a + bj , here a represents the real part and b represents the complex part. Both j and J are allowed but internally Python stores j.

>>> a=8+4j
>>> a=8+4J
>>> a
(8+4j)

There must not be any space between b and j

>>> a=9+5 j
SyntaxError: invalid syntax

2. Boolean Values: There are two Literals True and False which are known as Boolean Values.

>>> a=True
>>> a
True

>>> b=False
>>> b
False

3. Collections: Python provides 4 different types of collections:

• List : Comma separated list of items (may be of same / different type) enclosed in square brackets [ and ]
  Example: [“gargs” ,”academy”, 9467863365]
• Tuple : Comma separated list of items (may be of same / different type) enclosed in paranthesis ( and )
  Example: (“gargs” ,”academy”, 9467863365)
• Set : Comma separated list of unordered and unique items (may be of same / different type) enclosed in braces { and }
  Example: {“gargs” ,”academy”, 9467863365}
• String :  Any combination of characters or text written in single, double, or triple quotes. Internally strings are stored in single quotes.

>>> list1=["gargs" ,"academy", 9467863365]
>>> list1
['gargs', 'academy', 9467863365]

>>> tuple1=("gargs" ,"academy", 9467863365)
>>> tuple1
('gargs', 'academy', 9467863365)

>>> set1={"gargs" ,"academy", 9467863365}
>>> set1
{'academy', 9467863365, 'gargs'}

>>> str='gargs academy'
>>> str
'gargs academy'

>>> str="gargs academy"
>>> str
'gargs academy'

Triple quotes ”’ and “”” are used for multi line strings.

>>> str="""gargs
academy"""
>>> str
'gargs\nacademy'
>>> str='''gargs
academy'''
>>> str
'gargs\nacademy'

For multi line strings, statement continuation mark \ is used. If we want space in between the strings, we must give space explicitly.

>>> str="""gargs\
academy"""
>>> str
'gargsacademy'
>>> str="""gargs\
 academy"""
>>> str
'gargs academy'

It can also include binary, decimal, octal, hexadecimal integer values.

>>> a=bin(10)
>>> a
'0b1010'
>>> a=oct(10)
>>> a
'0o12'
>>> a=hex(10)
>>> a
'0xa'

4. Mapping/ Dictionary: Comma separated list of key : value paired form of unordered items (may be of same / different type) enclosed in braces { and }
Example: {“gargs” : 1 , 2 : “academy”, “phone” : 9467863365}

>>> dict1= {"gargs" : 1 , 2 : "academy", "phone" : 9467863365}
>>> dict1
{'gargs': 1, 2: 'academy', 'phone': 9467863365}

5. Special Value None: None is used to define a null or missing value.

>>> a=None
>>> a
>>> print(a)
None

Python supports assigning multiple values to variables in a single statement as:

>>> a,b,c=10,20,30
>>> a
10
>>> b
20
>>> c
30

>>> (a,b,c)=10,20,30
>>> a
10
>>> b
20
>>> c
30

>>> a,b,c=(10,20,30)
>>> a
10
>>> b
20
>>> c
30

>>> a,b,c=[10,20,30]
>>> a
10
>>> b
20
>>> c
30

>>> a,b,c={10,20,30}
>>> a
10
>>> b
20
>>> c
30

>>> a=10;b=20
>>> a=10,b=17
SyntaxError: cannot assign to literal
>>> a
10
>>> b
20

But if the count of l-values and r-values doesn’t match, python will report an error as:

>>> a,b=(1,2,3)
Traceback (most recent call last):
  File "<pyshell#21>", line 1, in <module>
    a,b=(1,2,3)
ValueError: too many values to unpack (expected 2)

We can assign all the remaining values in a list form in a variable by placing an * before the variable name as:

>>> a,*b=(1,2,3)
>>> a
1
>>> b
[2, 3]

We can also combine two or more statements in python as:

>>> a=10;b=20

>>> a
10
>>> b
20

But we can’s use comma to separate two or more expressions as:

>>> a=10,b=20
SyntaxError: cannot assign to literal