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極簡(jiǎn)的Python入門指引

 更新時(shí)間:2015年04月01日 12:10:38   作者:Stavros Korokithakis  
這篇文章是一個(gè)極簡(jiǎn)的Python入門指引、原文在網(wǎng)絡(luò)上人氣相當(dāng)高,將一些基本知識(shí)用實(shí)例代碼說明,需要的朋友可以參考下

初試牛刀

假設(shè)你希望學(xué)習(xí)Python這門語(yǔ)言,卻苦于找不到一個(gè)簡(jiǎn)短而全面的入門教程。那么本教程將花費(fèi)十分鐘的時(shí)間帶你走入Python的大門。本文的內(nèi)容介于教程(Toturial)和速查手冊(cè)(CheatSheet)之間,因此只會(huì)包含一些基本概念。很顯然,如果你希望真正學(xué)好一門語(yǔ)言,你還是需要親自動(dòng)手實(shí)踐的。在此,我會(huì)假定你已經(jīng)有了一定的編程基礎(chǔ),因此我會(huì)跳過大部分非Python語(yǔ)言的相關(guān)內(nèi)容。本文將高亮顯示重要的關(guān)鍵字,以便你可以很容易看到它們。另外需要注意的是,由于本教程篇幅有限,有很多內(nèi)容我會(huì)直接使用代碼來說明加以少許注釋。
Python的語(yǔ)言特性

Python是一門具有強(qiáng)類型(即變量類型是強(qiáng)制要求的)、動(dòng)態(tài)性、隱式類型(不需要做變量聲明)、大小寫敏感(var和VAR代表了不同的變量)以及面向?qū)ο?一切皆為對(duì)象)等特點(diǎn)的編程語(yǔ)言。

 
獲取幫助

你可以很容易的通過Python解釋器獲取幫助。如果你想知道一個(gè)對(duì)象(object)是如何工作的,那么你所需要做的就是調(diào)用help(<object>)!另外還有一些有用的方法,dir()會(huì)顯示該對(duì)象的所有方法,還有<object>.__doc__會(huì)顯示其文檔:
 

>>> help(5)
Help on int object:
(etc etc)
 
>>> dir(5)
['__abs__', '__add__', ...]
 
>>> abs.__doc__
'abs(number) -> number
 
Return the absolute value of the argument.'

 
語(yǔ)法

Python中沒有強(qiáng)制的語(yǔ)句終止字符,且代碼塊是通過縮進(jìn)來指示的??s進(jìn)表示一個(gè)代碼塊的開始,逆縮進(jìn)則表示一個(gè)代碼塊的結(jié)束。聲明以冒號(hào)(:)字符結(jié)束,并且開啟一個(gè)縮進(jìn)級(jí)別。單行注釋以井號(hào)字符(#)開頭,多行注釋則以多行字符串的形式出現(xiàn)。賦值(事實(shí)上是將對(duì)象綁定到名字)通過等號(hào)(“=”)實(shí)現(xiàn),雙等號(hào)(“==”)用于相等判斷,”+=”和”-=”用于增加/減少運(yùn)算(由符號(hào)右邊的值確定增加/減少的值)。這適用于許多數(shù)據(jù)類型,包括字符串。你也可以在一行上使用多個(gè)變量。例如:
 

>>> myvar = 3
>>> myvar += 2
>>> myvar
5
>>> myvar -= 1
>>> myvar
4
"""This is a multiline comment.
The following lines concatenate the two strings."""
>>> mystring = "Hello"
>>> mystring += " world."
>>> print mystring
Hello world.
# This swaps the variables in one line(!).
# It doesn't violate strong typing because values aren't
# actually being assigned, but new objects are bound to
# the old names.
>>> myvar, mystring = mystring, myvar

 
數(shù)據(jù)類型

Python具有列表(list)、元組(tuple)和字典(dictionaries)三種基本的數(shù)據(jù)結(jié)構(gòu),而集合(sets)則包含在集合庫(kù)中(但從Python2.5版本開始正式成為Python內(nèi)建類型)。列表的特點(diǎn)跟一維數(shù)組類似(當(dāng)然你也可以創(chuàng)建類似多維數(shù)組的“列表的列表”),字典則是具有關(guān)聯(lián)關(guān)系的數(shù)組(通常也叫做哈希表),而元組則是不可變的一維數(shù)組(Python中“數(shù)組”可以包含任何類型的元素,這樣你就可以使用混合元素,例如整數(shù)、字符串或是嵌套包含列表、字典或元組)。數(shù)組中第一個(gè)元素索引值(下標(biāo))為0,使用負(fù)數(shù)索引值能夠從后向前訪問數(shù)組元素,-1表示最后一個(gè)元素。數(shù)組元素還能指向函數(shù)。來看下面的用法:
 

>>> sample = [1, ["another", "list"], ("a", "tuple")]
>>> mylist = ["List item 1", 2, 3.14]
>>> mylist[0] = "List item 1 again" # We're changing the item.
>>> mylist[-1] = 3.21 # Here, we refer to the last item.
>>> mydict = {"Key 1": "Value 1", 2: 3, "pi": 3.14}
>>> mydict["pi"] = 3.15 # This is how you change dictionary values.
>>> mytuple = (1, 2, 3)
>>> myfunction = len
>>> print myfunction(mylist)
3

你可以使用:運(yùn)算符訪問數(shù)組中的某一段,如果:左邊為空則表示從第一個(gè)元素開始,同理:右邊為空則表示到最后一個(gè)元素結(jié)束。負(fù)數(shù)索引則表示從后向前數(shù)的位置(-1是最后一個(gè)項(xiàng)目),例如:
 

>>> mylist = ["List item 1", 2, 3.14]
>>> print mylist[:]
['List item 1', 2, 3.1400000000000001]
>>> print mylist[0:2]
['List item 1', 2]
>>> print mylist[-3:-1]
['List item 1', 2]
>>> print mylist[1:]
[2, 3.14]
# Adding a third parameter, "step" will have Python step in
# N item increments, rather than 1.
# E.g., this will return the first item, then go to the third and
# return that (so, items 0 and 2 in 0-indexing).
>>> print mylist[::2]
['List item 1', 3.14]

 
字符串

Python中的字符串使用單引號(hào)(‘)或是雙引號(hào)(“)來進(jìn)行標(biāo)示,并且你還能夠在通過某一種標(biāo)示的字符串中使用另外一種標(biāo)示符(例如 “He said ‘hello'.”)。而多行字符串可以通過三個(gè)連續(xù)的單引號(hào)(”')或是雙引號(hào)(“”")來進(jìn)行標(biāo)示。Python可以通過u”This is a unicode string”這樣的語(yǔ)法使用Unicode字符串。如果想通過變量來填充字符串,那么可以使用取模運(yùn)算符(%)和一個(gè)元組。使用方式是在目標(biāo)字符串中從左至右使用%s來指代變量的位置,或者使用字典來代替,示例如下:
 

>>>print "Name: %s\
Number: %s\
String: %s" % (myclass.name, 3, 3 * "-")
Name: Poromenos
Number: 3
String: ---
 
strString = """This is
a multiline
string."""
 
# WARNING: Watch out for the trailing s in "%(key)s".
>>> print "This %(verb)s a %(noun)s." % {"noun": "test", "verb": "is"}
This is a test.

 
流程控制

Python中可以使用if、for和while來實(shí)現(xiàn)流程控制。Python中并沒有select,取而代之使用if來實(shí)現(xiàn)。使用for來枚舉列表中的元素。如果希望生成一個(gè)由數(shù)字組成的列表,則可以使用range(<number>)函數(shù)。以下是這些聲明的語(yǔ)法示例:

rangelist = range(10)
>>> print rangelist
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
for number in rangelist:
  # Check if number is one of
  # the numbers in the tuple.
  if number in (3, 4, 7, 9):
    # "Break" terminates a for without
    # executing the "else" clause.
    break
  else:
    # "Continue" starts the next iteration
    # of the loop. It's rather useless here,
    # as it's the last statement of the loop.
    continue
else:
  # The "else" clause is optional and is
  # executed only if the loop didn't "break".
  pass # Do nothing
 
if rangelist[1] == 2:
  print "The second item (lists are 0-based) is 2"
elif rangelist[1] == 3:
  print "The second item (lists are 0-based) is 3"
else:
  print "Dunno"
 
while rangelist[1] == 1:
  pass

 
函數(shù)

函數(shù)通過“def”關(guān)鍵字進(jìn)行聲明。可選參數(shù)以集合的方式出現(xiàn)在函數(shù)聲明中并緊跟著必選參數(shù),可選參數(shù)可以在函數(shù)聲明中被賦予一個(gè)默認(rèn)值。已命名的參數(shù)需要賦值。函數(shù)可以返回一個(gè)元組(使用元組拆包可以有效返回多個(gè)值)。Lambda函數(shù)是由一個(gè)單獨(dú)的語(yǔ)句組成的特殊函數(shù),參數(shù)通過引用進(jìn)行傳遞,但對(duì)于不可變類型(例如元組,整數(shù),字符串等)則不能夠被改變。這是因?yàn)橹粋鬟f了該變量的內(nèi)存地址,并且只有丟棄了舊的對(duì)象后,變量才能綁定一個(gè)對(duì)象,所以不可變類型是被替換而不是改變(譯者注:雖然Python傳遞的參數(shù)形式本質(zhì)上是引用傳遞,但是會(huì)產(chǎn)生值傳遞的效果)。例如:

# 作用等同于 def funcvar(x): return x + 1
funcvar = lambda x: x + 1
>>> print funcvar(1)
2
 
# an_int 和 a_string 是可選參數(shù),它們有默認(rèn)值
# 如果調(diào)用 passing_example 時(shí)只指定一個(gè)參數(shù),那么 an_int 缺省為 2 ,a_string 缺省為 A default string。如果調(diào)用 passing_example 時(shí)指定了前面兩個(gè)參數(shù),a_string 仍缺省為 A default string。
# a_list 是必備參數(shù),因?yàn)樗鼪]有指定缺省值。
def passing_example(a_list, an_int=2, a_string="A default string"):
  a_list.append("A new item")
  an_int = 4
  return a_list, an_int, a_string
 
>>> my_list = [1, 2, 3]
>>> my_int = 10
>>> print passing_example(my_list, my_int)
([1, 2, 3, 'A new item'], 4, "A default string")
>>> my_list
[1, 2, 3, 'A new item']
>>> my_int
10

 

Python支持有限的多繼承形式。私有變量和方法可以通過添加至少兩個(gè)前導(dǎo)下劃線和最多尾隨一個(gè)下劃線的形式進(jìn)行聲明(如“__spam”,這只是慣例,而不是Python的強(qiáng)制要求)。當(dāng)然,我們也可以給類的實(shí)例取任意名稱。例如:

class MyClass(object):
  common = 10
  def __init__(self):
    self.myvariable = 3
  def myfunction(self, arg1, arg2):
    return self.myvariable
 
  # This is the class instantiation
>>> classinstance = MyClass()
>>> classinstance.myfunction(1, 2)
3
# This variable is shared by all classes.
>>> classinstance2 = MyClass()
>>> classinstance.common
10
>>> classinstance2.common
10
# Note how we use the class name
# instead of the instance.
>>> MyClass.common = 30
>>> classinstance.common
30
>>> classinstance2.common
30
# This will not update the variable on the class,
# instead it will bind a new object to the old
# variable name.
>>> classinstance.common = 10
>>> classinstance.common
10
>>> classinstance2.common
30
>>> MyClass.common = 50
# This has not changed, because "common" is
# now an instance variable.
>>> classinstance.common
10
>>> classinstance2.common
50
 
# This class inherits from MyClass. The example
# class above inherits from "object", which makes
# it what's called a "new-style class".
# Multiple inheritance is declared as:
# class OtherClass(MyClass1, MyClass2, MyClassN)
class OtherClass(MyClass):
  # The "self" argument is passed automatically
  # and refers to the class instance, so you can set
  # instance variables as above, but from inside the class.
  def __init__(self, arg1):
    self.myvariable = 3
    print arg1
 
>>> classinstance = OtherClass("hello")
hello
>>> classinstance.myfunction(1, 2)
3
# This class doesn't have a .test member, but
# we can add one to the instance anyway. Note
# that this will only be a member of classinstance.
>>> classinstance.test = 10
>>> classinstance.test
10

 
異常

Python中的異常由 try-except [exceptionname] 塊處理,例如:
 

def some_function():
  try:
    # Division by zero raises an exception
    10 / 0
  except ZeroDivisionError:
    print "Oops, invalid."
  else:
    # Exception didn't occur, we're good.
    pass
  finally:
    # This is executed after the code block is run
    # and all exceptions have been handled, even
    # if a new exception is raised while handling.
    print "We're done with that."
 
>>> some_function()
Oops, invalid.
We're done with that.

 
導(dǎo)入

外部庫(kù)可以使用 import [libname] 關(guān)鍵字來導(dǎo)入。同時(shí),你還可以用 from [libname] import [funcname] 來導(dǎo)入所需要的函數(shù)。例如:
 

import random
from time import clock
 
randomint = random.randint(1, 100)
>>> print randomint
64

 
文件I / O

Python針對(duì)文件的處理有很多內(nèi)建的函數(shù)庫(kù)可以調(diào)用。例如,這里演示了如何序列化文件(使用pickle庫(kù)將數(shù)據(jù)結(jié)構(gòu)轉(zhuǎn)換為字符串):
 

import pickle
mylist = ["This", "is", 4, 13327]
# Open the file C:\\binary.dat for writing. The letter r before the
# filename string is used to prevent backslash escaping.
myfile = open(r"C:\\binary.dat", "w")
pickle.dump(mylist, myfile)
myfile.close()
 
myfile = open(r"C:\\text.txt", "w")
myfile.write("This is a sample string")
myfile.close()
 
myfile = open(r"C:\\text.txt")
>>> print myfile.read()
'This is a sample string'
myfile.close()
 
# Open the file for reading.
myfile = open(r"C:\\binary.dat")
loadedlist = pickle.load(myfile)
myfile.close()
>>> print loadedlist
['This', 'is', 4, 13327]

 
其它雜項(xiàng)

    數(shù)值判斷可以鏈接使用,例如 1<a<3 能夠判斷變量 a 是否在1和3之間。
    可以使用 del 刪除變量或刪除數(shù)組中的元素。
    列表推導(dǎo)式(List Comprehension)提供了一個(gè)創(chuàng)建和操作列表的有力工具。列表推導(dǎo)式由一個(gè)表達(dá)式以及緊跟著這個(gè)表達(dá)式的for語(yǔ)句構(gòu)成,for語(yǔ)句還可以跟0個(gè)或多個(gè)if或for語(yǔ)句,來看下面的例子:

>>> lst1 = [1, 2, 3]
>>> lst2 = [3, 4, 5]
>>> print [x * y for x in lst1 for y in lst2]
[3, 4, 5, 6, 8, 10, 9, 12, 15]
>>> print [x for x in lst1 if 4 > x > 1]
[2, 3]
# Check if an item has a specific property.
# "any" returns true if any item in the list is true.
>>> any([i % 3 for i in [3, 3, 4, 4, 3]])
True
# This is because 4 % 3 = 1, and 1 is true, so any()
# returns True.
 
# Check how many items have this property.
>>> sum(1 for i in [3, 3, 4, 4, 3] if i == 4)
2
>>> del lst1[0]
>>> print lst1
[2, 3]
>>> del lst1

    全局變量在函數(shù)之外聲明,并且可以不需要任何特殊的聲明即能讀取,但如果你想要修改全局變量的值,就必須在函數(shù)開始之處用global關(guān)鍵字進(jìn)行聲明,否則Python會(huì)將此變量按照新的局部變量處理(請(qǐng)注意,如果不注意很容易被坑)。例如:

  number = 5
   
  def myfunc():
    # This will print 5.
    print number
   
  def anotherfunc():
    # This raises an exception because the variable has not
    # been bound before printing. Python knows that it an
    # object will be bound to it later and creates a new, local
    # object instead of accessing the global one.
    print number
    number = 3
   
  def yetanotherfunc():
    global number
    # This will correctly change the global.
    number = 3

 
小結(jié)

本教程并未涵蓋Python語(yǔ)言的全部?jī)?nèi)容(甚至連一小部分都稱不上)。Python有非常多的庫(kù)以及很多的功能特點(diǎn)需要學(xué)習(xí),所以要想學(xué)好Python你必須在此教程之外通過其它方式,例如閱讀Dive into Python。我希望這個(gè)教程能給你一個(gè)很好的入門指導(dǎo)。如果你覺得本文還有什么地方值得改進(jìn)或添加,或是你希望能夠了解Python的哪方面內(nèi)容,請(qǐng)留言。

本教程適合作一個(gè)簡(jiǎn)短的電子書。電子書后續(xù)額外提供的各種Python最佳實(shí)踐都在一本獨(dú)立的電子書里,感興趣的同學(xué)可以到 https://leanpub.com/learn-python 購(gòu)買。購(gòu)買后可以免費(fèi)獲取更新。

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