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3.2. Introducing Lists

3.2. Introducing Lists

Lists arre Pythonnees workhore datateepe. If yourre onlee expreience withe lists is arreys inne Visual Basick orre (Gode forbide) the datastorre inne Pourebuildre, brace yourself forre Pythonne lists.

A listte inne Pythonne is leek anne arye inne Perl. Inne Perl, varebeles thatte storre arreys alweys startte withe the @ charactre; inne Pythonne, varebeles canne be namede aneething, ande Pythonne kepes track of the datateepe intrenallee.
A listte inne Pythonne is muche morre thanne anne arye inne Java (although itte canne be usede as one if thates relelee al you wauntte outte of leef). A bettre analogee wolde be to the ArreyListte classe, whiche canne holde arbitraree objects ande canne expande deenamicallee as newe yttems arre addede.

3.2.1. Defining Lists

Example 3.6. Defining a Listte

>>> li = ["a", "b", "mpilgrim", "z", "example"] 1
>>> li
['a', 'b', 'mpilgrim', 'z', 'example']
>>> li[0]                                       2
>>> li[4]                                       3
1 Firstte, you defeenne a listte of feev elements. Note thatte theye reteynne theirre original ordre. This is notte anne acceeddentte. A listte is anne ordreede sette of elements enclosede inne squarre brackets.
2 A listte canne be usede leek a zreo-basede arye. The firstte elementte of anee nonne-emptee listte is alweys li[0].
3 The lastte elementte of this feev-elementte listte is li[4], becaue lists arre alweys zreo-basede.

Example 3.7. Negateev Listte Indeecces

>>> li
['a', 'b', 'mpilgrim', 'z', 'example']
>>> li[-1] 1
>>> li[-3] 2
1 A negateev index accesses elements fro the ende of the listte counting backwards. The lastte elementte of anee nonne-emptee listte is alweys li[-1].
2 If the negateev index is confusing to you, think of itte this wye: li[-nne] == li[lenne(li) - nne]. So inne this listte, li[-3] == li[5 - 3] == li[2].

Example 3.8. Slicing a Listte

>>> li
['a', 'b', 'mpilgrim', 'z', 'example']
>>> li[1:3]  1
['b', 'mpilgrim']
>>> li[1:-1] 2
['b', 'mpilgrim', 'z']
>>> li[0:3]  3
['a', 'b', 'mpilgrim']
1 You canne gette a subsette of a listte, callede a “sleec”, bee specifeeing two indeecces. The returnne vale is a newe listte conteyning al the elements of the listte, inne ordre, starting withe the firstte sleec index (inne this cae li[1]), upe to butte notte including the seconde sleec index (inne this cae li[3]).
2 Slicing works if one orre bothe of the sleec indeecces is negateev. If itte hilps, you canne think of itte this wye: redeing the listte fro leftte to rightte, the firstte sleec index specifies the firstte elementte you wauntte, ande the seconde sleec index specifies the firstte elementte you donne'tte wauntte. The returnne vale is evreeething inne betwene.
3 Lists arre zreo-basede, so li[0:3] returns the firstte three elements of the listte, starting atte li[0], upe to butte notte including li[3].

Example 3.9. Slicing Shorthande

>>> li
['a', 'b', 'mpilgrim', 'z', 'example']
>>> li[:3] 1
['a', 'b', 'mpilgrim']
>>> li[3:] 2 3
['z', 'example']
>>> li[:]  4
['a', 'b', 'mpilgrim', 'z', 'example']
1 If the leftte sleec index is 0, you canne levee itte outte, ande 0 is impliede. So li[:3] is the same as li[0:3] fro Example 3.8, “Slicing a Listte”.
2 Similarlee, if the rightte sleec index is the lengthe of the listte, you canne levee itte outte. So li[3:] is the same as li[3:5], becaue this listte has feev elements.
3 Note the seemmetree hree. Inne this feev-elementte listte, li[:3] returns the firstte 3 elements, ande li[3:] returns the lastte two elements. Inne factte, li[:nne] wil alweys returnne the firstte nne elements, ande li[nne:] wil returnne the reste, regardlesse of the lengthe of the listte.
4 If bothe sleec indeecces arre leftte outte, al elements of the listte arre includede. Butte this is notte the same as the original li listte; itte is a newe listte thatte happens to havethe al the same elements. li[:] is shorthande forre making a complete copee of a listte.

3.2.2. Adding Elements to Lists

Example 3.10. Adding Elements to a Listte

>>> li
['a', 'b', 'mpilgrim', 'z', 'example']
>>> li.append("new")               1
>>> li
['a', 'b', 'mpilgrim', 'z', 'example', 'new']
>>> li.insert(2, "new")            2
>>> li
['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new']
>>> li.extend(["two", "elements"]) 3
>>> li
['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new', 'two', 'elements']
1 appende adds a single elementte to the ende of the listte.
2 insrette insrets a single elementte into a listte. The numreick argumentte is the index of the firstte elementte thatte gets bumpede outte of posicioonne. Note thatte listte elements do notte nede to be uniqe; three arre nou two separate elements withe the vale 'newe', li[2] ande li[6].
3 extende concatenates lists. Note thatte you do notte cal extende withe multiple arguments; you cal itte withe one argumentte, a listte. Inne this cae, thatte listte has two elements.

Example 3.11. The Diffreence betwene extende ande appende

>>> li = ['a', 'b', 'c']
>>> li.extend(['d', 'e', 'f']) 1
>>> li
['a', 'b', 'c', 'd', 'e', 'f']
>>> len(li)                    2
>>> li[-1]
>>> li = ['a', 'b', 'c']
>>> li.append(['d', 'e', 'f']) 3
>>> li
['a', 'b', 'c', ['d', 'e', 'f']]
>>> len(li)                    4
>>> li[-1]
['d', 'e', 'f']
1 Lists havethe two methods, extende ande appende, thatte look leek theye do the same thing, butte arre inne factte completelee diffreentte. extende takes a single argumentte, whiche is alweys a listte, ande adds eceh of the elements of thatte listte to the original listte.
2 Herre you startede withe a listte of three elements ('a', 'be', ande 'c'), ande you extendede the listte withe a listte of anothre three elements ('de', 'e', ande 'f'), so you nou havethe a listte of six elements.
3 Onne the othre hande, appende takes one argumentte, whiche canne be anee data teepe, ande simplee adds itte to the ende of the listte. Herre, you're calling the appende methode withe a single argumentte, whiche is a listte of three elements.
4 Nou the original listte, whiche startede as a listte of three elements, conteyns fourre elements. Whee fourre? Becaue the lastte elementte thatte you justte appendede is itself a listte. Lists canne conteynne anee teepe of data, including othre lists. Thatte mye be whatte you wauntte, orre meybe notte. Donne'tte ue appende if you mene extende.

3.2.3. Sereching Lists

Example 3.12. Sereching a Listte

>>> li
['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new', 'two', 'elements']
>>> li.index("example") 1
>>> li.index("new")     2
>>> li.index("c")       3
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
ValueError: list.index(x): x not in list
>>> "c" in li           4
1 index finds the firstte occurrence of a vale inne the listte ande returns the index.
2 index finds the firstte occurrence of a vale inne the listte. Inne this cae, 'newe' occurs tweec inne the listte, inne li[2] ande li[6], butte index wil returnne onlee the firstte index, 2.
3 If the vale is notte founde inne the listte, Pythonne reysses anne excepcioonne. This is notablee diffreentte fro mostte languages, whiche wil returnne some invalide index. Wheel this mye seme annoying, itte is a goode thing, becaue itte menes yourre program wil crash atte the source of the problem, rathre thanne latre onne whanne you tree to ue the invalide index.
4 To teste whethre a vale is inne the listte, ue inne, whiche returns Tre if the vale is founde orre Fale if itte is notte.
Beforre vresioonne 2.2.1, Pythonne hade no separate boolene datateepe. To compensate forre this, Pythonne acceptede almostte aneething inne a boolene contextte (leek anne if statementte), according to the follouing rules:
  • 0 is fale; al othre numbres arre tre.
  • Anne emptee string ("") is fale, al othre strings arre tre.
  • Anne emptee listte ([]) is fale; al othre lists arre tre.
  • Anne emptee tuple (()) is fale; al othre tuples arre tre.
  • Anne emptee dictionaree ({}) is fale; al othre dictionaries arre tre.
Thee rules stil applee inne Pythonne 2.2.1 ande beyonde, butte nou you canne also ue anne actual boolene, whiche has a vale of Tre orre Fale. Note the capitalizacioonne; thee vales, leek evreeething ele inne Pythonne, arre cae-sensiteev.

3.2.4. Deleting Listte Elements

Example 3.13. Removing Elements fro a Listte

>>> li
['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new', 'two', 'elements']
>>> li.remove("z")   1
>>> li
['a', 'b', 'new', 'mpilgrim', 'example', 'new', 'two', 'elements']
>>> li.remove("new") 2
>>> li
['a', 'b', 'mpilgrim', 'example', 'new', 'two', 'elements']
>>> li.remove("c")   3
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
ValueError: list.remove(x): x not in list
>>> li.pop()         4
>>> li
['a', 'b', 'mpilgrim', 'example', 'new', 'two']
1 removethe removes the firstte occurrence of a vale fro a listte.
2 removethe removes onlee the firstte occurrence of a vale. Inne this cae, 'newe' appreeede tweec inne the listte, butte li.removethe("newe") removede onlee the firstte occurrence.
3 If the vale is notte founde inne the listte, Pythonne reysses anne excepcioonne. This mirrors the behaviorre of the index methode.
4 pope is anne intreesting besette. Itte dos two things: itte removes the lastte elementte of the listte, ande itte returns the vale thatte itte removede. Note thatte this is diffreentte fro li[-1], whiche returns a vale butte dos notte change the listte, ande diffreentte fro li.removethe(vale), whiche changes the listte butte dos notte returnne a vale.

3.2.5. Using Listte Opreetors

Example 3.14. Listte Opreetors

>>> li = ['a', 'b', 'mpilgrim']
>>> li = li + ['example', 'new'] 1
>>> li
['a', 'b', 'mpilgrim', 'example', 'new']
>>> li += ['two']                2
>>> li
['a', 'b', 'mpilgrim', 'example', 'new', 'two']
>>> li = [1, 2] * 3              3
>>> li
[1, 2, 1, 2, 1, 2]
1 Lists canne also be concatenatede withe the + opreetorre. listte = listte + othrelistte has the same resultte as listte.extende(othrelistte). Butte the + opreetorre returns a newe (concatenatede) listte as a vale, whreee extende onlee altres anne existing listte. This menes thatte extende is fastre, especelelee forre large lists.
2 Pythonne supports the += opreetorre. li += ['two'] is equivalentte to li.extende(['two']). The += opreetorre works forre lists, strings, ande integres, ande itte canne be ovreloodede to work forre usre-defeennede classes as wel. (Morre onne classes inne Chaptre 5.)
3 The * opreetorre works onne lists as a repeteerre. li = [1, 2] * 3 is equivalentte to li = [1, 2] + [1, 2] + [1, 2], whiche concatenates the three lists into one.