Variables and Types: One of the most powerful features of a
programming language is the ability to
manipulate variables. When we create a program, we often
like to store values so that it can be used later.
We use objects (variables) to capture data, which then can
be manipulated by computer to provide
information. By now, we know that object/variable is a name
which refers to a value.
Every object has:
An Identity,
A type, and
A value.
A. Identity of the object is its address in memory and does
not get change once it is created. We may know
it by typing id (variable)
We would be referring to objects as variable for now.
B. Type (i.e data type) is a set of values, and the
allowable operations on those values. It can be one of the
following:
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Computer Science
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1. Number: Number data type stores Numerical Values. This
data type is immutable i.e. value of its
object cannot be changed. Numbers are of three different
types:
Integer & Long (to store whole numbers i.e. decimal
digits without fraction part)
Float/floating point (to store numbers with fraction part)
Complex (to store real and imaginary part)
2. None: This is special data type with a single value. It
is used to signify the absence of value/false in a
situation. It is represented by None.
3. Sequence: A sequence is an ordered collection of items,
indexed by positive integers. It is a
combination of mutable (a mutable variable is one, whose
value may change) and immutable (an
immutable variable is one, whose value may not change) data
types. There are three types of sequence
data type available in Python, they are Strings, Lists &
Tuples.
3.1 String- is an ordered sequence of letters/characters.
They are enclosed in single quotes (' ') or
double quotes ('' "). The quotes are not part of
string. They only tell the computer about where the
string constant begins and ends. They can have any character
or sign, including space in them.
These are immutable. A string with length 1 represents a
character in Python.
3.2 Lists: List is also a sequence of values of any type.
Values in the list are called elements / items.
These are mutable and indexed/ordered. List is enclosed in
square brackets ([]).
3.3 Tuples: Tuples are a sequence of values of any type and
are indexed by integers. They are
immutable. Tuples are enclosed in ().
4. Sets: Set is unordered collection of values of any type
with no duplicate entry. It is immutable.
5. Mapping: This data type is unordered and mutable.
Dictionaries fall under Mappings.
5.1 Dictionaries: It can store any number of python objects.
What they store is a key -value pairs,
which are accessed using key. Dictionary is enclosed in
curly brackets ({}).
C. Value: Value is any number or a letter or string. To bind
value to a variable, we use assignment
operator (=).
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Data Types
Numbers None Sequences Sets Mappings
Dictionary Integer Strings Tuple List
Boolean
Floating Complex
Point
Computer Science
Keywords - are used to give some special meaning to the
interpreter and are used by Python
interpreter to recognize the structure of program.
A partial list of keywords in Python 2.7 is
and del from not
while as elif global
or with assert else
if pass Yield break
except import print class
exec in Raise continue
finally is return def
for lambda try
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