Friday, 25 March 2016

Python Variables and Datatypes

Python Variables

A variable is a location in memory used to store some data (value). They are given unique names to differentiate between different memory locations. The rules for writing a variable name is same as the rules for writing identifiers in Python.
We don't need to declare a variable before using it. In Python, we simply assign a value to a variable and it will exist. We don't even have to declare the type of the variable. This is handled internally according to the type of value we assign to the variable.

Variable assignment

We use the assignment operator (=) to assign values to a variable. Any type of value can be assigned to any valid variable.

a = 5
b = 3.2
c = "Hello"
Here, we have three assignment statements. 5 is an integer assigned to the variable a. Similarly,3.2 is a floating point number and "Hello" is a string (sequence of characters) assigned to the variables b and c respectively.

Multiple assignments

In Python, multiple assignments can be made in a single statement as follows:

a, b, c = 5, 3.2, "Hello"
If we want to assign the same value to multiple variables at once, we can do this as

x = y = z = "same"
This assigns the "same" string to all the three variables.

Datatypes in Python

Every value in Python has a datatype. Since everything is an object in Python programming, datatypes are actually classes and variables are instance (object) of these classes. There are various datatypes in Python. Some of the important types are listed below.

Python Numbers

Integers, floating point numbers and complex numbers falls under Python numbers category. They are defined as intfloat and complex class in Python. We can use the type() function to know which class a variable or a value belongs to and the isinstance() function to check if an object belongs to a particular class.

>>> a = 5
>>> type(a)
<class 'int'>
>>> type(2.0)
<class 'float'>
>>> isinstance(1+2j,complex)
Integers can be of any length, it is only limited by the memory available. A floating point number is accurate up to 15 decimal places. Integer and floating points are separated by decimal points. 1 is integer, 1.0 is floating point number. Complex numbers are written in the form, x + yj, where x is the real part and y is the imaginary part. Here are some examples.

>>> a = 1234567890123456789
>>> a
>>> b = 0.1234567890123456789
>>> b
>>> c = 1+2j
>>> c
Notice that the float variable b got truncated.

Python List

List is an ordered sequence of items. It is one of the most used datatype in Python and is very flexible. All the items in a list do not need to be of the same type. Declaring a list is pretty straight forward. Items separated by commas are enclosed within brackets [ ].

>>> a = [1, 2.2, 'python']
>>> type(a)
<class 'list'>
We can use the slicing operator [ ] to extract an item or a range of items from a list. Index starts form 0 in Python.

>>> a = [5,10,15,20,25,30,35,40]
>>> a[2]
>>> a[0:3]
[5, 10, 15]
>>> a[5:]
[30, 35, 40]
Lists are mutable, meaning, value of elements of a list can be altered.

>>> a = [1,2,3]
>>> a[2]=4
>>> a
[1, 2, 4]

Python Tuple

Tuple is an ordered sequence of items same as list. The only difference is that tuples are immutable. Tuples once created cannot be modified. They are used to write-protect data and are usually faster than list as it cannot change dynamically. Tuple is defined within parentheses () where items are separated by commas.

>>> t = (5,'program', 1+3j)
>>> type(t)
<class 'tuple'>
We can use the slicing operator [] to extract items but we cannot change its value.

>>> t[1]
>>> t[0:3]
(5, 'program', (1+3j))
>>> t[0] = 10
Traceback (most recent call last):
  File "<string>", line 301, in runcode
  File "<interactive input>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

Python Strings

String is sequence of Unicode characters. We can use single quotes or double quotes to represent strings. Multi-line strings can be denoted using triple quotes, ''' or """.

>>> s = "This is a string"
>>> type(s)
<class 'str'>
>>> s = '''a multiline
... string'''
Like list and tuple, slicing operator [ ] can be used with string. Strings are immutable.

>>> s = 'Hello world!'
>>> s[4]
>>> s[6:11]
>>> s[5] ='d'
Traceback (most recent call last):
  File "<string>", line 301, in runcode
  File "<interactive input>", line 1, in <module>
TypeError: 'str' object does not support item assignment

Python Set

Set is an unordered collection of unique items. Set is defined by values separated by comma inside braces { }. Items in a set are not ordered.

>>> a = {5,2,3,1,4}
>>> a
{1, 2, 3, 4, 5}
>>> type(a)
<class 'set'>
We can perform set operations like union, intersection on two sets. Set have unique values. They eliminate duplicates.

>>> a = {1,2,2,3,3,3}
>>> a
{1, 2, 3}
Since, set are unordered collection, indexing has no meaning. Hence the slicing operator [] does not work.

>>> a = {1,2,3}
>>> a[1]
Traceback (most recent call last):
  File "<string>", line 301, in runcode
  File "<interactive input>", line 1, in <module>
TypeError: 'set' object does not support indexing

Python Dictionary

Dictionary is an unordered collection of key-value pairs. It is generally used when we have a huge amount of data. Dictionaries are optimized for retrieving data. We must know the key to retrieve the value. In Python, dictionaries are defined within braces {} with each item being a pair in the form key:value. Key and value can be of any type.

>>> d = {1:'value','key':2}
>>> type(d)
<class 'dict'>
We use key to retrieve the respective value. But not the other way around.

>>> d[1]
>>> d['key']
>>> d[2]
Traceback (most recent call last):
  File "<string>", line 301, in runcode
  File "<interactive input>", line 1, in <module>
KeyError: 2

Conversion between datatypes

We can convert between different datatypes by using different type conversion functions like int(), float(), str() etc.

>>> float(5)
Conversion from float to int will truncate the value (make it closer to zero).

>>> int(10.6)
>>> int(-10.6)
Conversion to and from string must contain compatible values.

>>> float('2.5')
>>> str(25)
>>> int('1p')
Traceback (most recent call last):
  File "<string>", line 301, in runcode
  File "<interactive input>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '1p'
We can even convert one sequence to another.

>>> set([1,2,3])
{1, 2, 3}
>>> tuple({5,6,7})
(5, 6, 7)
>>> list('hello')
['h', 'e', 'l', 'l', 'o']
To convert to dictionary, each element must be a pair

>>> dict([[1,2],[3,4]])
{1: 2, 3: 4}
>>> dict([(3,26),(4,44)])
{3: 26, 4: 44}

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