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Safari | Python Developer's Handbook -> Built-In Data Types





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Built-In Data Types

Built-in data types are types that are already built into the interpreter. They are divided into two groups:

Immutable Data Types

These objects cannot have their values altered (for example, strings, numbers, and tuples).

Mutable Data Types

These objects can have their values manipulated (for example, lists and dictionaries).

Sometimes, it becomes necessary to assign a null value to a variable using the special data type known as None:

						
>>> x = 1
>>> x
1
>>> print x
1
>>> x = None
>>> x
>>>

					

As you could see, nothing was returned. However, if you try to print this value, the print method of the object will specially handle the None value by returning a None result. This is shown in the following:

						
>>> print x
None
				
					

Numbers

Python provides the following numeric data types: integer, floating-point, hexadecimal (base 16), and octal (base 8). Some examples of these data types are 43, 1.5, 0xB3, and 045, respectively.

Tip

Hexadecimal numbers must always be preceded by 0x, and octal numbers must be preceded by 0.



Python can do a lot of things with numbers:

It can write equations:

						
>>> 3*(3.0/34)
0.264705882353

					

It can use functions:

						
>>> round(12.32,1)
12.3

					

It can make comparisons:

						
>>> x = 2
>>> 0<x<5
1

					

It can make binary operations, such as shifting and masking:

						
>>> 16<<2
64
>>> 40&0xab
40
>>> 2|1
3
>>> ~2
-3
>>> 3^4
7

					

A very important detail is the fact that Python truncates integer divisions:

						
>>> 3/2
1

					

If you really want the decimals, you have two options. Either you pass a converted number to the division function, or you put a decimal point in your number, as illustrated here:

						
>>> x = 3
>>> float(x)/2
1.5
>>> x
3
>>> 3.0/2
1.5

					

Python supports long integerswith unlimited size. To let Python know that it should handle an integer as a long integer, you need to put an L at the end of the number:

						
>>> 2L**100
1267650600228229401496703205376L
				
					

Otherwise you get an error message:

						
>>> 2**100
Traceback (innermost last):
  File "<stdin>", line 1, in ?
OverflowError: integer pow()
				
					

Chapter 4, "Exception Handling," teaches you how to interpret this exception message.

Python also handles complex numbers in the format (real part + imaginary part):

						
>>> 2j**2
(-4+0j)
				
					

Strings

Python considers a string as a sequence of characters. Therefore, every time you use, for example, the string "Parrot", internally Python handles it as the sequence ["P", "a", "r", "r", "o", "t"]. The first indexer value is always the number zero. Hence, to have access to the letter P, you need to say "Parrot"[0] and to access the letter a, you need to say "Parrot"[1]. Using the same concept, we can get access to all the other elements.

The following is an example of string operators:

						
>>> "dead parrot " + "sketch"  # concatenation
"dead parrot sketch"
>>> "parrot " * 2              # repetition
"parrot parrot"
>>> "parrot"[1]                # indexing
"a"
>>> "parrot"[-1]               # indexing backward
"t"
>>> "parrot"[1:3]              # slicing (*)
"ar"

					

When slicing, it isn't necessary to include both first and last elements. Whenever you omit one of the elements, it is assumed that you want everything in that direction. Note that the second argument is always a positional reference.

						
>>> "parrot"[1:]
"arrot"
>>> "parrot"[:3]
"par"

					

Always remember that assigning z = x doesn't make a copy of the object x. Instead, it creates a new reference for that object (as you already saw in the earlier round example). If you have to create a copy of a sequence named x, you need to type:

						
>>> z = x[:]

					

The variable z will identify the middle of the variable x, and it will be initialized with everything from the left direction plus everything from the right direction. Note that since Python 1.5, id(s) == id(s[:]) for strings because of string interning.

Strings cannot be modified after creation. It isn't possible to assign a value to a substring because strings are immutable. See the error message in the next example:

						
>>> t = "pxrrot"
>>> t[1:2] = "a"
Traceback (innermost last):
  File "<stdin>", line 1, in ?
TypeError: object doesn't support slice assignment

					

In cases like this, the usual solution is a little trick:

						
s = s[:left_element] + new_substring + s[right_element:]

					

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