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Python Tutorial | |
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This chapter describes some things you've learned about already in more detail, and adds some new things as well.
本章節(jié)深入講述一些你已經(jīng)學(xué)習(xí)過(guò)的東西，并且還加入了新的內(nèi)容。
The list data type has some more methods. Here are all of the methods of list objects:
鏈表類(lèi)型有很多方法，這里是鏈表類(lèi)型的所有方法：
| x) |
a[len(a):] = [x].
把一個(gè)元素添加到鏈表的結(jié)尾，相當(dāng)于 a[len(a):] = [x]
| L) |
a[len(a):] = L.
通過(guò)添加指定鏈表的所有元素來(lái)擴(kuò)充鏈表，相當(dāng)于 a[len(a):] = L。
| i, x) |
a.insert(0, x)
inserts at the front of the list, and a.insert(len(a), x)
is equivalent to a.append(x).
在指定位置插入一個(gè)元素。第一個(gè)參數(shù)是準(zhǔn)備插入到其前面的那個(gè)元素的索引，例如a.insert(0, x) 會(huì)插入到整個(gè)鏈表之前，而a.insert(len(a), x) 相當(dāng)于 a.append(x)。
| x) |
刪除鏈表中值為x的第一個(gè)元素。如果沒(méi)有這樣的元素，就會(huì)返回一個(gè)錯(cuò)誤。
| [i]) |
a.pop() returns the last item in the
list. The item is also removed from the list. (The square brackets
around the i in the method signature denote that the parameter
is optional, not that you should type square brackets at that
position. You will see this notation frequently in the
Python Library Reference.)
從鏈表的指定位置刪除元素，并將其返回。如果沒(méi)有指定索引，a.pop()返回最后一個(gè)元素。元素隨即從鏈表中被刪除。（方法中i兩邊的方括號(hào)表示這個(gè)參數(shù)是可選的，而不是要求你輸入一對(duì)方括號(hào)，你會(huì)經(jīng)常在Python 庫(kù)參考手冊(cè)中遇到這樣的標(biāo)記。）
| x) |
返回鏈表中第一個(gè)值為x的元素的索引。如果沒(méi)有匹配的元素就會(huì)返回一個(gè)錯(cuò)誤。
| x) |
返回x在鏈表中出現(xiàn)的次數(shù)。
| ) |
對(duì)鏈表中的元素進(jìn)行適當(dāng)?shù)呐判颉?
| ) |
倒排鏈表中的元素。
An example that uses most of the list methods:
下面這個(gè)示例演示了鏈表的大部分方法：
>>> a = [66.6, 333, 333, 1, 1234.5]
>>> print a.count(333), a.count(66.6), a.count('x')
2 1 0
>>> a.insert(2, -1)
>>> a.append(333)
>>> a
[66.6, 333, -1, 333, 1, 1234.5, 333]
>>> a.index(333)
1
>>> a.remove(333)
>>> a
[66.6, -1, 333, 1, 1234.5, 333]
>>> a.reverse()
>>> a
[333, 1234.5, 1, 333, -1, 66.6]
>>> a.sort()
>>> a
[-1, 1, 66.6, 333, 333, 1234.5]
The list methods make it very easy to use a list as a stack, where the last element added is the first element retrieved (``last-in, first-out''). To add an item to the top of the stack, use append(). To retrieve an item from the top of the stack, use pop() without an explicit index. For example:
鏈表方法使得鏈表可以很方便的做為一個(gè)堆棧來(lái)使用，堆棧作為特定的數(shù)據(jù)結(jié)構(gòu)，最先進(jìn)入的元素最后一個(gè)被釋放（后進(jìn)先出）。用append() 方法可以把一個(gè)元素添加到堆棧頂。用不指定索引的pop() 方法可以把一個(gè)元素從堆棧頂釋放出來(lái)。例如：
>>> stack = [3, 4, 5] >>> stack.append(6) >>> stack.append(7) >>> stack [3, 4, 5, 6, 7] >>> stack.pop() 7 >>> stack [3, 4, 5, 6] >>> stack.pop() 6 >>> stack.pop() 5 >>> stack [3, 4]
You can also use a list conveniently as a queue, where the first
element added is the first element retrieved (``first-in,
first-out''). To add an item to the back of the queue, use
append(). To retrieve an item from the front of the queue,
use pop() with 0 as the index. For example:
你也可以把鏈表當(dāng)做隊(duì)列使用，隊(duì)列作為特定的數(shù)據(jù)結(jié)構(gòu)，最先進(jìn)入的元素最先釋放（先進(jìn)先出）。使用 append()方法可以把元素添加到隊(duì)列最后，以0為參數(shù)調(diào)用 pop() 方法可以把最先進(jìn)入的元素釋放出來(lái)。例如：
>>> queue = ["Eric", "John", "Michael"]
>>> queue.append("Terry") # Terry arrives
>>> queue.append("Graham") # Graham arrives
>>> queue.pop(0)
'Eric'
>>> queue.pop(0)
'John'
>>> queue
['Michael', 'Terry', 'Graham']
There are three built-in functions that are very useful when used with lists: filter(), map(), and reduce().
對(duì)于鏈表來(lái)講，有三個(gè)內(nèi)置函數(shù)非常有用：filter()， map()， 和 reduce()。
"filter(function, sequence)" returns a sequence (of
the same type, if possible) consisting of those items from the
sequence for which function(item) is true. For
example, to compute some primes:
"filter(function, sequence)"返回一個(gè)序列（sequence），包括了給定序列中所有調(diào)用function(item)后返回值為true的元素。（如果可能的話，會(huì)返回相同的類(lèi)型）。例如，以下程序可以計(jì)算部分素?cái)?shù)：
>>> def f(x): return x % 2 != 0 and x % 3 != 0 ... >>> filter(f, range(2, 25)) [5, 7, 11, 13, 17, 19, 23]
"map(function, sequence)" calls
function(item) for each of the sequence's items and
returns a list of the return values. For example, to compute some
cubes:
"map(function, sequence)" 為每一個(gè)元素依次調(diào)用function(item)并將返回值組成一個(gè)鏈表返回。例如，以下程序計(jì)算立方：
>>> def cube(x): return x*x*x ... >>> map(cube, range(1, 11)) [1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
More than one sequence may be passed; the function must then have as
many arguments as there are sequences and is called with the
corresponding item from each sequence (or None if some sequence
is shorter than another). For example:
可以傳入多個(gè)序列，函數(shù)也必須要有對(duì)應(yīng)數(shù)量的參數(shù)，執(zhí)行時(shí)會(huì)依次用各序列上對(duì)應(yīng)的元素來(lái)調(diào)用函數(shù)（如果某些序列比其它的短，就用None來(lái)代替）。如果把None做為一個(gè)函數(shù)傳入，則直接返回參數(shù)做為替代。例如：
>>> seq = range(8) >>> def add(x, y): return x+y ... >>> map(add, seq, seq) [0, 2, 4, 6, 8, 10, 12, 14]
"reduce(func, sequence)" returns a single value constructed by calling the binary function func on the first two items of the sequence, then on the result and the next item, and so on. For example, to compute the sum of the numbers 1 through 10:
"reduce(func, sequence)" 返回一個(gè)單值，它是這樣構(gòu)造的：首先以序列的前兩個(gè)元素調(diào)用函數(shù)，再以返回值和第三個(gè)參數(shù)調(diào)用，依次執(zhí)行下去。例如，以下程序計(jì)算1到10的整數(shù)之和：
>>> def add(x,y): return x+y ... >>> reduce(add, range(1, 11)) 55
If there's only one item in the sequence, its value is returned; if the sequence is empty, an exception is raised.
如果序列中只有一個(gè)元素，就返回它，如果序列是空的，就拋出一個(gè)異常。
A third argument can be passed to indicate the starting value. In this case the starting value is returned for an empty sequence, and the function is first applied to the starting value and the first sequence item, then to the result and the next item, and so on. For example,
可以傳入第三個(gè)參數(shù)做為初始值。如果序列是空的，就返回初始值，否則函數(shù)會(huì)先接收初始值和序列的第一個(gè)元素，然后是返回值和下一個(gè)元素，依此類(lèi)推。例如：
>>> def sum(seq): ... def add(x,y): return x+y ... return reduce(add, seq, 0) ... >>> sum(range(1, 11)) 55 >>> sum([]) 0
Don't use this example's definition of sum(): since summing
numbers is such a common need, a built-in function
sum(sequence) is already provided, and works exactly like
this.
New in version 2.3.
不要像示例中這樣定義sum()：因?yàn)楹嫌?jì)數(shù)值是一個(gè)通用的需求，在2.3版中，提供了內(nèi)置的sum(sequence) 函數(shù)。
New in version 2.3.
List comprehensions provide a concise way to create lists without resorting to use of map(), filter() and/or lambda. The resulting list definition tends often to be clearer than lists built using those constructs. Each list comprehension consists of an expression followed by a for clause, then zero or more for or if clauses. The result will be a list resulting from evaluating the expression in the context of the for and if clauses which follow it. If the expression would evaluate to a tuple, it must be parenthesized.
鏈表推導(dǎo)式提供了一個(gè)創(chuàng)建鏈表的簡(jiǎn)單途徑，無(wú)需使用map()， filter() 以及 lambda。返回鏈表的定義通常要比創(chuàng)建這些鏈表更清晰。每一個(gè)鏈表推導(dǎo)式包括在一個(gè)for 語(yǔ)句之后的表達(dá)式，零或多個(gè) for或 if 語(yǔ)句。返回值是由 for 或 if子句之后的表達(dá)式得到的元素組成的鏈表。如果想要得到一個(gè)元組，必須要加上括號(hào)。
>>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
>>> [weapon.strip() for weapon in freshfruit]
['banana', 'loganberry', 'passion fruit']
>>> vec = [2, 4, 6]
>>> [3*x for x in vec]
[6, 12, 18]
>>> [3*x for x in vec if x > 3]
[12, 18]
>>> [3*x for x in vec if x < 2]
[]
>>> [[x,x**2] for x in vec]
[[2, 4], [4, 16], [6, 36]]
>>> [x, x**2 for x in vec] # error - parens required for tuples
File "<stdin>", line 1, in ?
[x, x**2 for x in vec]
^
SyntaxError: invalid syntax
>>> [(x, x**2) for x in vec]
[(2, 4), (4, 16), (6, 36)]
>>> vec1 = [2, 4, 6]
>>> vec2 = [4, 3, -9]
>>> [x*y for x in vec1 for y in vec2]
[8, 6, -18, 16, 12, -36, 24, 18, -54]
>>> [x+y for x in vec1 for y in vec2]
[6, 5, -7, 8, 7, -5, 10, 9, -3]
>>> [vec1[i]*vec2[i] for i in range(len(vec1))]
[8, 12, -54]
List comprehensions are much more flexible than map() and can be applied to functions with more than one argument and to nested functions:
鏈表推導(dǎo)式比 map()更復(fù)雜，可調(diào)用多個(gè)參數(shù)和嵌套函數(shù)。
>>> [str(round(355/113.0, i)) for i in range(1,6)] ['3.1', '3.14', '3.142', '3.1416', '3.14159']
There is a way to remove an item from a list given its index instead of its value: the del statement. This can also be used to remove slices from a list (which we did earlier by assignment of an empty list to the slice). For example:
有一個(gè)方法可從鏈表中刪除指定索引的元素：del 語(yǔ)句。這個(gè)方法也可以從鏈表中刪除切片（之前我們是把一個(gè)空鏈表賦給切片）。例如：
>>> a = [-1, 1, 66.6, 333, 333, 1234.5] >>> del a[0] >>> a [1, 66.6, 333, 333, 1234.5] >>> del a[2:4] >>> a [1, 66.6, 1234.5]
del can also be used to delete entire variables:
del 也可以用于刪除整個(gè)變量：
>>> del a
Referencing the name a hereafter is an error (at least until
another value is assigned to it). We'll find other uses for
del later.
此后再引用這個(gè)名字會(huì)發(fā)生錯(cuò)誤（至少要到給它賦另一個(gè)值為止）。后面我們還會(huì)發(fā)現(xiàn)del的其它用法。
We saw that lists and strings have many common properties, such as indexing and slicing operations. They are two examples of sequence data types. Since Python is an evolving language, other sequence data types may be added. There is also another standard sequence data type: the tuple.
我們知道鏈表和字符串有很多通用的屬性，例如索引和切片操作。它們是序列類(lèi)型中的兩種。因?yàn)镻ython是一個(gè)在不停進(jìn)化的語(yǔ)言，也可能會(huì)加入其它的
As you see, on output tuples are alway enclosed in parentheses, so
that nested tuples are interpreted correctly; they may be input with
or without surrounding parentheses, although often parentheses are
necessary anyway (if the tuple is part of a larger expression).
如你所見(jiàn)，元組在輸出時(shí)總是有括號(hào)的，以便于正確表達(dá)嵌套結(jié)構(gòu)。在輸入時(shí)可能有或沒(méi)有括號(hào)都可以，不過(guò)經(jīng)常括號(hào)都是必須的（如果元組是一個(gè)更大的表達(dá)式的一部分）。
Tuples have many uses. For example: (x, y) coordinate pairs, employee
records from a database, etc. Tuples, like strings, are immutable: it
is not possible to assign to the individual items of a tuple (you can
simulate much of the same effect with slicing and concatenation,
though). It is also possible to create tuples which contain mutable
objects, such as lists.
元組有很多用途。例如(x, y)坐標(biāo)點(diǎn)，數(shù)據(jù)庫(kù)中的員工記錄等等。元組就像字符串，不可改變：不能給元組的一個(gè)獨(dú)立的元素賦值（盡管你可以通過(guò)聯(lián)接和切片來(lái)模仿）。也可以通過(guò)包含可變對(duì)象來(lái)創(chuàng)建元組，例如鏈表。
A special problem is the construction of tuples containing 0 or 1
items: the syntax has some extra quirks to accommodate these. Empty
tuples are constructed by an empty pair of parentheses; a tuple with
one item is constructed by following a value with a comma
(it is not sufficient to enclose a single value in parentheses).
Ugly, but effective. For example:
一個(gè)特殊的問(wèn)題是構(gòu)造包含零個(gè)或一個(gè)元素的元組：為了適應(yīng)這種情況，語(yǔ)法上有一些額外的改變。一對(duì)空的括號(hào)可以創(chuàng)建空元組；要?jiǎng)?chuàng)建一個(gè)單元素元組可以在值后面跟一個(gè)逗號(hào)（在括號(hào)中放入一個(gè)單值是不夠的）。丑陋，但是有效。例如：
The statement
語(yǔ)句 t = 12345, 54321, 'hello!' 是元組封裝（sequence packing）的一個(gè)例子：值 12345， 54321 和 'hello!' 被封裝進(jìn)元組。其逆操作可能是這樣：
This is called, appropriately enough, sequence unpacking.
Sequence unpacking requires that the list of variables on the left
have the same number of elements as the length of the sequence. Note
that multiple assignment is really just a combination of tuple packing
and sequence unpacking!
這個(gè)調(diào)用被稱為序列拆封非常合適。序列拆封要求左側(cè)的變量數(shù)目與序列的元素個(gè)數(shù)相同。要注意的是可變參數(shù)（multiple
assignment ）其實(shí)只是元組封裝和序列拆封的一個(gè)結(jié)合！
There is a small bit of asymmetry here: packing multiple values
always creates a tuple, and unpacking works for any sequence.
這里有一點(diǎn)不對(duì)稱：封裝多重參數(shù)通常會(huì)創(chuàng)建一個(gè)元組，而拆封操作可以作用于任何序列。
Another useful data type built into Python is the
dictionary.
Dictionaries are sometimes found in other languages as ``associative
memories'' or ``associative arrays''. Unlike sequences, which are
indexed by a range of numbers, dictionaries are indexed by keys,
which can be any immutable type; strings and numbers can always be
keys. Tuples can be used as keys if they contain only strings,
numbers, or tuples; if a tuple contains any mutable object either
directly or indirectly, it cannot be used as a key. You can't use
lists as keys, since lists can be modified in place using their
append() and extend() methods, as well as slice and
indexed assignments.
另一個(gè)非常有用的Python內(nèi)建數(shù)據(jù)類(lèi)型是字典。字典在某些語(yǔ)言中可能稱為“聯(lián)合內(nèi)存”（``associative memories''）或“聯(lián)合數(shù)組”（``associative arrays''）。序列是以連續(xù)的整數(shù)為索引，與此不同的是，字典以關(guān)鍵字為索引，關(guān)鍵字可以是任意不可變類(lèi)型，通常用字符串或數(shù)值。如果元組中只包含字符串和數(shù)字，它可以做為關(guān)鍵字，如果它直接或間接的包含了可變對(duì)象，就不能當(dāng)做關(guān)鍵字。不能用鏈表做關(guān)鍵字，因?yàn)殒湵砜梢杂盟鼈兊?tt class="method">append() 和 extend()方法，或者用切片、或者通過(guò)檢索變量來(lái)即時(shí)改變。
It is best to think of a dictionary as an unordered set of
key: value pairs, with the requirement that the keys are unique
(within one dictionary).
A pair of braces creates an empty dictionary:
理解字典的最佳方式是把它看做無(wú)序的關(guān)鍵字：值 對(duì)（key:value pairs）集合，關(guān)鍵字必須是互不相同的（在同一個(gè)字典之內(nèi)）。一對(duì)大括號(hào)創(chuàng)建一個(gè)空的字典：
The main operations on a dictionary are storing a value with some key
and extracting the value given the key. It is also possible to delete
a key:value pair
with
字典的主要操作是依據(jù)關(guān)鍵字來(lái)存儲(chǔ)和析取值。也可以用
The keys() method of a dictionary object returns a list of all
the keys used in the dictionary, in random order (if you want it
sorted, just apply the sort() method to the list of keys). To
check whether a single key is in the dictionary, use the
has_key() method of the dictionary.
字典的 keys()方法返回由所有關(guān)鍵字組成的鏈表，該鏈表的順序不定（如果你需要它有序，只能調(diào)用關(guān)鍵字鏈表的sort() 方法）。使用字典的 has_key()方法可以檢查字典中是否存在某一關(guān)鍵字。
Here is a small example using a dictionary:
這是一個(gè)關(guān)于字典應(yīng)用的小示例：
The dict() constructor builds dictionaries directly from
lists of key-value pairs stored as tuples. When the pairs form a
pattern, list comprehensions can compactly specify the key-value list.
鏈表中存儲(chǔ)關(guān)鍵字-值對(duì)元組的話，字典可以從中直接構(gòu)造。關(guān)鍵字-值對(duì)來(lái)自一個(gè)模式時(shí)，可以用鏈表推導(dǎo)式簡(jiǎn)單的表達(dá)關(guān)鍵字-值鏈表。
When looping through dictionaries, the key and corresponding value can
be retrieved at the same time using the iteritems() method.
在字典中循環(huán)時(shí)，關(guān)鍵字和對(duì)應(yīng)的值可以使用 iteritems()方法同時(shí)解讀出來(lái)。
When looping through a sequence, the position index and corresponding
value can be retrieved at the same time using the
enumerate() function.
在序列中循環(huán)時(shí)，索引位置和對(duì)應(yīng)值可以使用enumerate()函數(shù)同時(shí)得到。
To loop over two or more sequences at the same time, the entries
can be paired with the zip() function.
同時(shí)循環(huán)兩個(gè)或更多的序列，可以使用 zip() 整體解讀。
The conditions used in
用于
The comparison operators
Comparisons can be chained. For example,
比較操作可以傳遞。例如
Comparisons may be combined by the Boolean operators
比較操作可以通過(guò)邏輯操作符
The Boolean operators
邏輯操作符
It is possible to assign the result of a comparison or other Boolean
expression to a variable. For example,
可以把比較或其它邏輯表達(dá)式的返回值賦給一個(gè)變量，例如：
Note that in Python, unlike C, assignment cannot occur inside expressions.
C programmers may grumble about this, but it avoids a common class of
problems encountered in C programs: typing
需要注意的是Python與C不同，在表達(dá)式內(nèi)部不能賦值。C 程序員經(jīng)常對(duì)此抱怨，不過(guò)它避免了一類(lèi)在 C 程序中司空見(jiàn)慣的錯(cuò)誤：想要在解析式中使
Sequence objects may be compared to other objects with the same
sequence type. The comparison uses lexicographical ordering:
first the first two items are compared, and if they differ this
determines the outcome of the comparison; if they are equal, the next
two items are compared, and so on, until either sequence is exhausted.
If two items to be compared are themselves sequences of the same type,
the lexicographical comparison is carried out recursively. If all
items of two sequences compare equal, the sequences are considered
equal. If one sequence is an initial sub-sequence of the other, the
shorter sequence is the smaller (lesser) one. Lexicographical
ordering for strings uses the ASCII ordering for individual
characters. Some examples of comparisons between sequences with the
same types:
序列對(duì)象可以與相同類(lèi)型的其它對(duì)象比較。比較操作按 字典序 進(jìn)行：首先比較前兩個(gè)元素，如果不同，就決定了比較的結(jié)果；如果相同，就比較后兩個(gè)元素，依此類(lèi)推，直到所有序列都完成比較。如果兩個(gè)元素本身就是同樣類(lèi)型的序列，就遞歸字典序比較。如果兩個(gè)序列的所有子項(xiàng)都相等，就認(rèn)為序列相等。如果一個(gè)序列是另一個(gè)序列的初始子序列，較短的一個(gè)序列就小于另一個(gè)。字符串的字典序按照單字符的 ASCII 順序。下面是同類(lèi)型序列之間比較的一些例子：
Note that comparing objects of different types is legal. The outcome
is deterministic but arbitrary: the types are ordered by their name.
Thus, a list is always smaller than a string, a string is always
smaller than a tuple, etc. Mixed numeric types are compared according
to their numeric value, so 0 equals 0.0, etc.5.1
需要注意的是不同類(lèi)型的對(duì)象比較是合法的。輸出結(jié)果是確定而非任意的：類(lèi)型按它們的名字排序。因而，一個(gè)鏈表（list）總是小于一個(gè)字符串（string），一個(gè)字符串（string）總是小于一個(gè)元組（tuple）等等。數(shù)值類(lèi)型比較時(shí)會(huì)統(tǒng)一它們的數(shù)據(jù)類(lèi)型，所以0等于0.0，等等。5.2
>>> t = 12345, 54321, 'hello!'
>>> t[0]
12345
>>> t
(12345, 54321, 'hello!')
>>> # Tuples may be nested:
... u = t, (1, 2, 3, 4, 5)
>>> u
((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
>>> empty = ()
>>> singleton = 'hello', # <-- note trailing comma
>>> len(empty)
0
>>> len(singleton)
1
>>> singleton
('hello',)
t = 12345, 54321, 'hello!' is an example of
tuple packing: the values 12345, 54321 and
'hello!' are packed together in a tuple. The reverse operation
is also possible:
>>> x, y, z = t
5.4 Dictionaries 字典
{}.
Placing a comma-separated list of key:value pairs within the
braces adds initial key:value pairs to the dictionary; this is also the
way dictionaries are written on output.
{}。初始化鏈表時(shí)，在大括號(hào)內(nèi)放置一組逗號(hào)分隔的關(guān)鍵字：值對(duì)，這也是字典輸出的方式。
del.
If you store using a key that is already in use, the old value
associated with that key is forgotten. It is an error to extract a
value using a non-existent key.
del來(lái)刪除關(guān)鍵字：值對(duì)。如果你用一個(gè)已經(jīng)存在的關(guān)鍵字存儲(chǔ)值，以前為該關(guān)鍵字分配的值就會(huì)被遺忘。試圖析取從一個(gè)不存在的關(guān)鍵字中讀取值會(huì)導(dǎo)致錯(cuò)誤。
>>> tel = {'jack': 4098, 'sape': 4139}
>>> tel['guido'] = 4127
>>> tel
{'sape': 4139, 'guido': 4127, 'jack': 4098}
>>> tel['jack']
4098
>>> del tel['sape']
>>> tel['irv'] = 4127
>>> tel
{'guido': 4127, 'irv': 4127, 'jack': 4098}
>>> tel.keys()
['guido', 'irv', 'jack']
>>> tel.has_key('guido')
True
>>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
{'sape': 4139, 'jack': 4098, 'guido': 4127}
>>> dict([(x, x**2) for x in vec]) # use a list comprehension
{2: 4, 4: 16, 6: 36}
5.5 循環(huán)技巧 Looping Techniques
>>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
>>> for k, v in knights.iteritems():
... print k, v
...
gallahad the pure
robin the brave
>>> for i, v in enumerate(['tic', 'tac', 'toe']):
... print i, v
...
0 tic
1 tac
2 toe
>>> questions = ['name', 'quest', 'favorite color']
>>> answers = ['lancelot', 'the holy grail', 'blue']
>>> for q, a in zip(questions, answers):
... print 'What is your %s? It is %s.' % (q, a)
...
What is your name? It is lancelot.
What is your quest? It is the holy grail.
What is your favorite color? It is blue.
5.6 深入條件控制 More on Conditions
while and if statements above can
contain other operators besides comparisons.
while 和 if 語(yǔ)句的條件包括了比較之外的操作符。
in and not in check whether a value
occurs (does not occur) in a sequence. The operators is and
is not compare whether two objects are really the same object; this
only matters for mutable objects like lists. All comparison operators
have the same priority, which is lower than that of all numerical
operators.
in 和 not in 比較操作符審核值是否在一個(gè)區(qū)間之內(nèi)。操作符 is is not 和比較兩個(gè)對(duì)象是否相同；這只和諸如鏈表這樣的可變對(duì)象有關(guān)。所有的比較操作符具有相同的優(yōu)先級(jí)，低于所有的數(shù)值操作。
a < b == c tests
whether a is less than b and moreover b equals
c.
a < b == c 審核是否 a 小于 b 并 b 等于c。
and and
or, and the outcome of a comparison (or of any other Boolean
expression) may be negated with not. These all have lower
priorities than comparison operators again; between them, not has
the highest priority, and or the lowest, so that
A and not B or C is equivalent to (A and (not B)) or C. Of
course, parentheses can be used to express the desired composition.
and 和 or 組合，比較的結(jié)果可以用 not 來(lái)取反義。這些操作符的優(yōu)先級(jí)又低于比較操作符，在它們之中，not 具有最高的優(yōu)先級(jí)， or 優(yōu)先級(jí)最低，所以A and not B or C 等于 (A and (not B)) or C。當(dāng)然，表達(dá)式可以用期望的方式表示。
and and or are so-called
short-circuit operators: their arguments are evaluated from
left to right, and evaluation stops as soon as the outcome is
determined. For example, if A and C are true but
B is false, A and B and C does not evaluate the
expression C. In general, the return value of a short-circuit
operator, when used as a general value and not as a Boolean, is the
last evaluated argument.
and 和 or 也稱作短路操作符：它們的參數(shù)從左向右解析，一旦結(jié)果可以確定就停止。例如，如果 A 和 C 為真而 B 為假， A and B and C 不會(huì)解析 C。作用于一個(gè)普通的非邏輯值時(shí)，短路操作符的返回值通常是最后一個(gè)變量
>>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
>>> non_null = string1 or string2 or string3
>>> non_null
'Trondheim'
= in an expression when
== was intended.
== 時(shí)誤用了 = 操作符。
5.7 比較序列和其它類(lèi)型 Comparing Sequences and Other Types
(1, 2, 3) < (1, 2, 4)
[1, 2, 3] < [1, 2, 4]
'ABC' < 'C' < 'Pascal' < 'Python'
(1, 2, 3, 4) < (1, 2, 4)
(1, 2) < (1, 2, -1)
(1, 2, 3) == (1.0, 2.0, 3.0)
(1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4)
Footnotes
譯者：劉鑫(march.liu AT gmail DOT com) 由：limodou轉(zhuǎn)(limodou AT gmail DOT com) CHM 文件制作：Colin.Wang 2007年9月
Python Tutorial
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