Andrew Hudson - Fedora™ Unleashed, 2008 edition

>>> mystring = "hello"

>>> list3 = [1, mystring, 3]

>>> list3

[1, 'hello', 3]

>>> mystring = "world"

>>> list3

[1, 'hello', 3]

Of course, this raises the question of how you copy without references when references are the default. The answer, for lists, is that you use the [:] slice, which you saw earlier. This slices from the first element to the last, inclusive, essentially copying it without refer ences. Here is how that looks:

>>> list4 = ["a", "b", "c"]

>>> list5 = list4[:]

>>> list4 = list4 + ["d"]

>>> list5

['a', 'b', 'c']

>>> list4

['a', 'b', 'c', 'd']

Lists have their own collections of built-in methods, such as sort(), append(), and pop(). The latter two add and remove single elements from the end of the list, with pop() also returning the removed element. For example:

>>> list5 = ["nick", "paul", "julian", "graham"]

>>> list5.sort()

>>> list5

['graham', 'julian', 'nick', 'paul']

>>> list5.pop() 'paul'

>>> list5

['graham', 'julian', 'nick']

>>> list5.append("Rebecca")

In addition, one interesting method of strings returns a list: split(). This takes a character by which to split and then gives you a list in which each element is a chunk from the string. For example:

>>> string = "This is a test string";

>>> string.split(" ")

['This', 'is', 'a', 'test', 'string']

Lists are used extensively in Python, although this is slowly changing as the language matures.

Unlike lists, dictionaries are collections with no fixed order. Instead, they have a key (the name of the element) and a value (the content of the element), and Python places them wherever it needs to for maximum performance. When defining dictionaries, you need to use braces ( { }) and colons ( :). You start with an opening brace and then give each element a key and a value, separated by a colon, like this:

>>> mydict = { "perl" : "a language", "php" : "another language" }

>>> mydict

{'php': 'another language', 'perl': 'a language'}

This example has two elements, with keys perland php. However, when the dictionary is printed, we find that phpcomes before perl— Python hasn't respected the order in which they were entered. You can index into a dictionary using the normal code:

>>> mydict["perl"] 'a language'

However, because a dictionary has no fixed sequence, you cannot take a slice, or index by position.

Like lists, dictionaries are mutable and can also be nested; however, unlike lists, you cannot merge two dictionaries by using +. A key is used to locate dictionary elements, so having two elements with the same key would cause a clash. Instead, you should use the update()method, which merges two arrays by overwriting clashing keys.

You can also use the keys()method to return a list of all the keys in a dictionary.

So far, we have been looking at just data types, which should show you how powerful Python's data types are. However, you simply cannot write complex programs without conditional statements and loops.

Python has most of the standard conditional checks, such as >(greater than), <=(less than or equal to), and ==(equal), but it also adds some new ones, such as in. For example, you can use in to check whether a string or a list contains a given character/element:

>>> mystring = "J Random Hacker"

>>> "r" in mystring

True

>>> "Hacker" in mystring

True

>>> "hacker" in mystring

False

The last example demonstrates how inis case sensitive. You can use the operator for lists, too:

>>> mylist = ["soldier", "sailor", "tinker", "spy"]

>>> "tailor" in mylist

False

Other comparisons on these complex data types are done item by item:

>>> list1 = ["alpha", "beta", "gamma"]

>>> list2 = ["alpha", "beta", "delta"]

>>> list1 > list2

True

list1's first element ( alpha) is compared against list2's first element ( alpha) and, because they are equal, the next element is checked. That is equal also, so the third element is checked, which is different. The g in gammacomes after the d in deltain the alphabet, so gammais considered greater than deltaand list1is considered greater than list2.

Loops come in two types, and both are equally flexible. For example, the forloop can iterate through letters in a string or elements in a list:

>>> string = "Hello, Python!"

>>> for s in string: print s,

...

H e l l o , P y t h o n !

The forloop takes each letter in stringand assigns it to s. The letter is then printed to the screen when you use the print command, but note the comma at the end: this tells Python not to insert a line break after each letter. The "..." is there because Python allows you to enter more code in the loop; you need to press Enter again here to have the loop execute.

The same construct can be used for lists:

>>> mylist = ["andi", "rasmus", "zeev"]

>>> for p in mylist: print p

...

andi

rasmus

zeev

Without the comma after the printstatement, each item is printed on its own line. The other loop type is the whileloop, and it looks similar:

>> while 1: print "This will loop forever!"

...

This will loop forever!

This will loop forever!

This will loop forever!

This will loop forever!

This will loop forever!

(etc)

Traceback (most recent call last):

File "", line 1, in ?

KeyboardInterrupt

>>>

That is an infinite loop (it will carry on printing that text forever), so you have to press Ctrl+C to interrupt it and regain control.

If you want to use multiline loops, you need to get ready to use your Tab key: Python handles loop blocks by recording the level of indent used. Some people find this odious; others admire it for forcing clean coding on users. Most of us, though, just get on with programming!

For example:

>>> i = 0

>>> while i < 3:

... j = 0

... while j < 3:

... print "Pos: " + str(i) + "," + str(j) + ")"