Python Data Types Example

Python has five “standard” data types: numeric, string, list, tuple, and dictionary.

Numeric

Numeric data types are for storing numeric values. In Python 2.x, there are four types of numeric data, Integers (signed), Long (signed), Float (floating point numbers), and complex numbers. Booleans (0 or 1) are subtypes of integer under version 2.x. The actual range of values that can be stored in any different numeric type will vary from system to system. On my Windows machine, the maximum value for an integer is 2,147,483,647. You can find the actual maximum value for an integer on your system by using the
following code in a Python Interactive Shell session:

import sys
print sys.maxint
2147483647 (My system)

Hexadecimal, octal, and binary numbers also fall under the Numeric data type umbrella. Hexadecimal numbers are base 16. They range from 0 to 9 and A to F. Octal numbers are base 8 numbers. They range from 0 to 7. Binary numbers are base 2 numbers and only include 0 and 1. Examples of each follow:

Hexadecimal - 1B = 27 (decimal)
Octal - 033 = 27 (decimal)
Binary - 00011011 = 27 (decimal)

String

The string data type is a series of characters that is delimited by quotation marks (either single or double). Strings will be discussed seperately in Chapter 4. An example of strings would include:

'This is a string'
"3.14159"

Lists, Tuples, and Dictionaries, here is a quick description.

List

Lists are a way to create multiple values that are referenced by a single variable name with a zero-based index, similar to arrays in other programming languages. Lists use square brackets (“[ ]”) to define them. An example would be:

ColorList = ['Red','Orange','Yellow','Green','Blue','Purple']
ColorList[2] would be the value ‘Yellow.’

Tuple

Tuples are a number of values seperated by commas. The data within a tuple may consist
of numbers, strings, and even other objects:

t = 3,42,'The time has come for all good men'
Like lists, tuples are referenced by a zero-based index. t[1] would be the value “42”

Dictionary

A dictionary is like a mini database in memory. Each item in a dictionary has two parts:
a key and a value. Each item is referenced (usually) by its key. A dictionary uses curly brackets to define them:

dict = {"Fname":"Jack","LName":"Sprat"}

In this example, there are two sets of data in the ‘dict’ dictionary. ‘Fname’ is the key for the value ‘Jack’ and ‘LName’ is the key for the value ‘Sprat’

Data Type Conversion

There are several built-in functions to perform conversion from one data type to another.

int(s,[base])

Returns an integer from the string s. If base is provided, specifies what base to use for the conversion. If the string is not that of a value (binary, hex, octal, integer, long, etc.), you will receive an error.

>>> int('1001',2)
9
>>> int('FA',16)
250

long(s,[base])

Returns a long integer from the string s. If base is provided, it specifies what base to use for the conversion. If the string is not a valid number, it will return an error.

>>> long(30219568420)
30219568420L

float(s)
Returns a floating point number from the string s. If the string is not a valid number, this will return an error.

>>> d = '3.14159'
>>> e = float(d)
>>> e
3.14159

complex(real [,imaginary])

Creates a complex number.

>>> complex(3.14159)
(3.14159+0j)
>>> complex(-123.45)
(-123.45+0j)
>>> complex(32,43)
(32+43j)

str(x)

Returns a string based on the numeric value x. If (x) is not a valid number, this function will return an error.

>>> str(3.14159)
'3.14159'

tuple(s)

Returns a tuple based on sequence s. The sequence should be something like a string or a list.

>>> tuple([3,5,7])
(3, 5, 7)
>>> tuple('abcd')
('a', 'b', 'c', 'd')

list(s)

Returns a list based on sequence s. The sequence should be something like a string or a tuple.

>>> list((1,2,3))
[1, 2, 3]
>>> list('abcd')
['a', 'b', 'c', 'd']

set(l)

Returns a set based on sequence l. Sequence l must be a list.

>>> set([7,3,11])
set([11, 3, 7])

dict(s) (s must be sequence of (key,value) tuples)
Returns a dictionary from a list of tuples.
>>> dict([(‘fname’,’fred’),(‘value’,1)])
{‘value’: 1, ‘fname’: ‘fred’}

frozenset(s)

Returns a frozenset created from set s.

>>> s = set([7,3,1])
>>> frozenset(s)
frozenset([1, 3, 7])

chr(x)

Returns a character created from integer x in the range of 0 to 255.

>>> chr(65)
'A'

unichr(x)
Returns a unicode character created from integer x in the range (on most machines) of 0 to 0x10000.
>>> unichr(1000)
u’\u03e8′

ord(c)

Returns the ascii value of character c.

>>> ord('M')
77

 

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