4. 深入了解流程控制

除了刚刚介绍过的 while 语句，Python 中也会使用其他语言中常见的流程控制语句，只是稍有变化。

4.1. if 陳述式

或許最常見的陳述式種類就是 if 了。舉例來說：

>>> x = int(raw_input("Please enter an integer: "))
Please enter an integer: 42
>>> if x < 0:
...     x = 0
...     print 'Negative changed to zero'
... elif x == 0:
...     print 'Zero'
... elif x == 1:
...     print 'Single'
... else:
...     print 'More'
...
More

可以有零個或多個 elif 段落，且 else 段落可有可無。關鍵字 elif 只是「else if」的縮寫，並且用來避免過多的縮排。一個 if … elif … elif … 序列可以用來替代其他語言中出現的 switch 或 case 陳述式。

4.2. for 陳述式

在 Python 中的 for 陳述式可能不同於在 C 或 Pascal 中所看到的使用方式。與其只能疊代 (iterate) 一個等差級數（如 Pascal），或給與使用者定義疊代步進方式與結束條件（如 C），Python 的 for 陳述疊代任何序列（list 或者字串）的元素，以他們出現在序列中的順序。例如（無意雙關）：

>>> # Measure some strings:
... words = ['cat', 'window', 'defenestrate']
>>> for w in words:
...     print w, len(w)
...
cat 3
window 6
defenestrate 12

如果你在迴圈中需要修改一個你正在疊代的序列（例如重複一些選擇的元素），那麼會建議你先建立一個序列的拷貝。疊代序列並不暗示建立新的拷貝。此時 slice 語法就讓這件事十分容易完成：

>>> for w in words[:]:  # Loop over a slice copy of the entire list.
...     if len(w) > 6:
...         words.insert(0, w)
...
>>> words
['defenestrate', 'cat', 'window', 'defenestrate']

4.3. range() 函式

If you do need to iterate over a sequence of numbers, the built-in function range() comes in handy. It generates lists containing arithmetic progressions:

>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

The given end point is never part of the generated list; range(10) generates a list of 10 values, the legal indices for items of a sequence of length 10. It is possible to let the range start at another number, or to specify a different increment (even negative; sometimes this is called the 『step』):

>>> range(5, 10)
[5, 6, 7, 8, 9]
>>> range(0, 10, 3)
[0, 3, 6, 9]
>>> range(-10, -100, -30)
[-10, -40, -70]

欲疊代一個序列的索引值，你可以搭配使用 range() 和 len() 如下：

>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
>>> for i in range(len(a)):
...     print i, a[i]
...
0 Mary
1 had
2 a
3 little
4 lamb

然而，在多數的情況，使用 enumerate() 函式將更為方便，詳見迴圈技巧。

4.4. break 和 continue 陳述、迴圈內 else 段落

break 陳述，如同 C 語言，終止包含其最內部的 for 或 while 迴圈。

迴圈可以帶有一個 else 段落。當迴圈歷遍疊代的 list （在 for 中）或條件為偽（在 while 中）時，這個段落會被執行；但迴圈被 break 陳述終止時則不會。底下尋找質數的迴圈即示範了這個行為：

>>> for n in range(2, 10):
...     for x in range(2, n):
...         if n % x == 0:
...             print n, 'equals', x, '*', n/x
...             break
...     else:
...         # loop fell through without finding a factor
...         print n, 'is a prime number'
...
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3

（沒錯，這是正確的程式碼。請看仔細：else 段落屬於 for 迴圈，並非 if 陳述。）

當 else 段落與迴圈使用時，相較於搭配 if 陳述使用，它的行為與搭配 try 陳述使用時更為相似：try 的 else 段落在沒有任何例外 (exception) 時執行，而迴圈的 else 段落在沒有任何 break 時執行。更多有關 try 陳述和例外的介紹，見處理例外。

continue 陳述，亦承襲於 C 語言，讓所屬的迴圈繼續執行下個疊代：

>>> for num in range(2, 10):
...     if num % 2 == 0:
...         print "Found an even number", num
...         continue
...     print "Found a number", num
Found an even number 2
Found a number 3
Found an even number 4
Found a number 5
Found an even number 6
Found a number 7
Found an even number 8
Found a number 9

4.5. pass 陳述式

pass 陳述不執行任何動作。它用在語法上需要一個陳述但不需要執行任何動作的時候。例如：

>>> while True:
...     pass  # Busy-wait for keyboard interrupt (Ctrl+C)
...

這經常用於定義一個最簡單的類別：

>>> class MyEmptyClass:
...     pass
...

pass 亦可作為一個函式或條件判斷主體的預留位置，它可幫助你以更宏觀的角度思考並撰寫新的程式碼。pass 可被忽略：

>>> def initlog(*args):
...     pass   # Remember to implement this!
...

4.6. 定義函式 (function)

我們可以建立一個函式來產生費式數列到任何一個上界：

>>> def fib(n):    # write Fibonacci series up to n
...     """Print a Fibonacci series up to n."""
...     a, b = 0, 1
...     while a < n:
...         print a,
...         a, b = b, a+b
...
>>> # Now call the function we just defined:
... fib(2000)
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597

關鍵字 def 帶入一個函式的定義。它之後必須連著該函式的名稱和置於括號之中的參數。自下一行起，所有縮排的陳述成為該函式的主體。

一個函式的第一個陳述可以是一個字串值；此情況該字串值被視為該函式的說明文件字串，即 docstring。（關於 docstring 的細節請參見說明文件字串段落。）有些工具可以使用 docstring 來自動產生線上或可列印的文件，或讓使用者能自由地自原始碼中瀏覽文件。在原始碼中加入 docstring 是個好慣例，應該養成這樣的習慣。

函式執行期間會建立一個新的符號表（symbol table）來儲存該函式內的區域變數。更精確地說，所有在函式內的變數賦值都會把該值儲存在一個區域符號表。然而，在讀取一個變數時，會先從區域符號表起搜尋，其次為所有包含其函式的區域符號表，其次為全域符號表，最後為所有內建的名稱。因此，在函式中，全域變數無法被直接賦值（除非該變數有被 global 陳述句提及），但它們可以被讀取。

在一個函式被呼叫的時候，實際傳入的參數（引數）會被加入至該函數的區域符號表。因此，引數傳入的方式為傳值呼叫 (call by value)（這裡傳遞的「值」永遠是一個物件的參照（reference），而不是該物件的值）。1 當一個函式呼叫別的函式時，在被呼叫的函式中會建立一個新的區域符號表。

一個函式定義會把該函式名稱加入至當前的符號表。該函式名稱的值帶有一個型別，並被直譯器辨識為使用者自定函式（user-defined function）。該值可以被賦予給別的變數名，而該變數也可以被當作函式使用。這即是常見的重新命名方式：

>>> fib
<function fib at 10042ed0>
>>> f = fib
>>> f(100)
0 1 1 2 3 5 8 13 21 34 55 89

Coming from other languages, you might object that fib is not a function but a procedure since it doesn’t return a value. In fact, even functions without a return statement do return a value, albeit a rather boring one. This value is called None (it’s a built-in name). Writing the value None is normally suppressed by the interpreter if it would be the only value written. You can see it if you really want to using print:

>>> fib(0)
>>> print fib(0)
None

如果要寫一個函式回傳費式數列的 list 而不是直接印出它，這也很容易：

>>> def fib2(n):  # return Fibonacci series up to n
...     """Return a list containing the Fibonacci series up to n."""
...     result = []
...     a, b = 0, 1
...     while a < n:
...         result.append(a)    # see below
...         a, b = b, a+b
...     return result
...
>>> f100 = fib2(100)    # call it
>>> f100                # write the result
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

這個例子一樣示範了一些新的 Python 特性：

• return 陳述會讓一個函式回傳一個值。單獨使用 return 不外加一個表達式作為引數會回傳 None。一個函式執行到結束也會回傳 None。

• result.append(a) 陳述呼叫了一個 list 物件的 result method（方法）。method 為「屬於」一個物件的函式，命名規則為 obj.methodname，其中 obj 為某個物件（亦可為一表達式），而 methodname 為該 method 的名稱，並由該物件的型別所定義。不同的型別代表不同的 method。不同型別的 method 可以擁有一樣的名稱而不會讓 Python 混淆。（你可以使用 class 定義自己的物件型別和 method，見 Classes）這裡 append() method 定義在 list 物件中；它會加入一個新的元素在該 list 的末端。這個例子等同於 result = result + [a]，但更有效率。

4.7. More on Defining Functions

It is also possible to define functions with a variable number of arguments. There are three forms, which can be combined.

4.7.1. Default Argument Values

The most useful form is to specify a default value for one or more arguments. This creates a function that can be called with fewer arguments than it is defined to allow. For example:

def ask_ok(prompt, retries=4, complaint='Yes or no, please!'):
    while True:
        ok = raw_input(prompt)
        if ok in ('y', 'ye', 'yes'):
            return True
        if ok in ('n', 'no', 'nop', 'nope'):
            return False
        retries = retries - 1
        if retries < 0:
            raise IOError('refusenik user')
        print complaint

This function can be called in several ways:

• giving only the mandatory argument: ask_ok('Do you really want to quit?')

• giving one of the optional arguments: ask_ok('OK to overwrite the file?', 2)

• or even giving all arguments: ask_ok('OK to overwrite the file?', 2, 'Come on, only yes or no!')

This example also introduces the in keyword. This tests whether or not a sequence contains a certain value.

The default values are evaluated at the point of function definition in the defining scope, so that

i = 5

def f(arg=i):
    print arg

i = 6
f()

will print 5.

Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls:

def f(a, L=[]):
    L.append(a)
    return L

print f(1)
print f(2)
print f(3)

This will print

[1]
[1, 2]
[1, 2, 3]

If you don’t want the default to be shared between subsequent calls, you can write the function like this instead:

def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

4.7.2. Keyword Arguments

Functions can also be called using keyword arguments of the form kwarg=value. For instance, the following function:

def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'):
    print "-- This parrot wouldn't", action,
    print "if you put", voltage, "volts through it."
    print "-- Lovely plumage, the", type
    print "-- It's", state, "!"

accepts one required argument (voltage) and three optional arguments (state, action, and type). This function can be called in any of the following ways:

parrot(1000)                                          # 1 positional argument
parrot(voltage=1000)                                  # 1 keyword argument
parrot(voltage=1000000, action='VOOOOOM')             # 2 keyword arguments
parrot(action='VOOOOOM', voltage=1000000)             # 2 keyword arguments
parrot('a million', 'bereft of life', 'jump')         # 3 positional arguments
parrot('a thousand', state='pushing up the daisies')  # 1 positional, 1 keyword

but all the following calls would be invalid:

parrot()                     # required argument missing
parrot(voltage=5.0, 'dead')  # non-keyword argument after a keyword argument
parrot(110, voltage=220)     # duplicate value for the same argument
parrot(actor='John Cleese')  # unknown keyword argument

In a function call, keyword arguments must follow positional arguments. All the keyword arguments passed must match one of the arguments accepted by the function (e.g. actor is not a valid argument for the parrot function), and their order is not important. This also includes non-optional arguments (e.g. parrot(voltage=1000) is valid too). No argument may receive a value more than once. Here’s an example that fails due to this restriction:

>>> def function(a):
...     pass
...
>>> function(0, a=0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: function() got multiple values for keyword argument 'a'

When a final formal parameter of the form **name is present, it receives a dictionary (see Mapping Types — dict) containing all keyword arguments except for those corresponding to a formal parameter. This may be combined with a formal parameter of the form *name (described in the next subsection) which receives a tuple containing the positional arguments beyond the formal parameter list. (*name must occur before **name.) For example, if we define a function like this:

def cheeseshop(kind, *arguments, **keywords):
    print "-- Do you have any", kind, "?"
    print "-- I'm sorry, we're all out of", kind
    for arg in arguments:
        print arg
    print "-" * 40
    keys = sorted(keywords.keys())
    for kw in keys:
        print kw, ":", keywords[kw]

It could be called like this:

cheeseshop("Limburger", "It's very runny, sir.",
           "It's really very, VERY runny, sir.",
           shopkeeper='Michael Palin',
           client="John Cleese",
           sketch="Cheese Shop Sketch")

and of course it would print:

-- Do you have any Limburger ?
-- I'm sorry, we're all out of Limburger
It's very runny, sir.
It's really very, VERY runny, sir.
----------------------------------------
client : John Cleese
shopkeeper : Michael Palin
sketch : Cheese Shop Sketch

Note that the list of keyword argument names is created by sorting the result of the keywords dictionary’s keys() method before printing its contents; if this is not done, the order in which the arguments are printed is undefined.

4.7.3. Arbitrary Argument Lists

Finally, the least frequently used option is to specify that a function can be called with an arbitrary number of arguments. These arguments will be wrapped up in a tuple (see Tuples 和序列 (Sequences)). Before the variable number of arguments, zero or more normal arguments may occur.

def write_multiple_items(file, separator, *args):
    file.write(separator.join(args))

4.7.4. Unpacking Argument Lists

The reverse situation occurs when the arguments are already in a list or tuple but need to be unpacked for a function call requiring separate positional arguments. For instance, the built-in range() function expects separate start and stop arguments. If they are not available separately, write the function call with the *-operator to unpack the arguments out of a list or tuple:

>>> range(3, 6)             # normal call with separate arguments
[3, 4, 5]
>>> args = [3, 6]
>>> range(*args)            # call with arguments unpacked from a list
[3, 4, 5]

In the same fashion, dictionaries can deliver keyword arguments with the **-operator:

>>> def parrot(voltage, state='a stiff', action='voom'):
...     print "-- This parrot wouldn't", action,
...     print "if you put", voltage, "volts through it.",
...     print "E's", state, "!"
...
>>> d = {"voltage": "four million", "state": "bleedin' demised", "action": "VOOM"}
>>> parrot(**d)
-- This parrot wouldn't VOOM if you put four million volts through it. E's bleedin' demised !

4.7.5. Lambda Expressions

Small anonymous functions can be created with the lambda keyword. This function returns the sum of its two arguments: lambda a, b: a+b. Lambda functions can be used wherever function objects are required. They are syntactically restricted to a single expression. Semantically, they are just syntactic sugar for a normal function definition. Like nested function definitions, lambda functions can reference variables from the containing scope:

>>> def make_incrementor(n):
...     return lambda x: x + n
...
>>> f = make_incrementor(42)
>>> f(0)
42
>>> f(1)
43

The above example uses a lambda expression to return a function. Another use is to pass a small function as an argument:

>>> pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
>>> pairs.sort(key=lambda pair: pair[1])
>>> pairs
[(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]

4.7.6. 說明文件字串

There are emerging conventions about the content and formatting of documentation strings.

The first line should always be a short, concise summary of the object’s purpose. For brevity, it should not explicitly state the object’s name or type, since these are available by other means (except if the name happens to be a verb describing a function’s operation). This line should begin with a capital letter and end with a period.

If there are more lines in the documentation string, the second line should be blank, visually separating the summary from the rest of the description. The following lines should be one or more paragraphs describing the object’s calling conventions, its side effects, etc.

The Python parser does not strip indentation from multi-line string literals in Python, so tools that process documentation have to strip indentation if desired. This is done using the following convention. The first non-blank line after the first line of the string determines the amount of indentation for the entire documentation string. (We can’t use the first line since it is generally adjacent to the string’s opening quotes so its indentation is not apparent in the string literal.) Whitespace 「equivalent」 to this indentation is then stripped from the start of all lines of the string. Lines that are indented less should not occur, but if they occur all their leading whitespace should be stripped. Equivalence of whitespace should be tested after expansion of tabs (to 8 spaces, normally).

Here is an example of a multi-line docstring:

>>> def my_function():
...     """Do nothing, but document it.
...
...     No, really, it doesn't do anything.
...     """
...     pass
...
>>> print my_function.__doc__
Do nothing, but document it.

    No, really, it doesn't do anything.

4.8. Intermezzo: Coding Style

Now that you are about to write longer, more complex pieces of Python, it is a good time to talk about coding style. Most languages can be written (or more concise, formatted) in different styles; some are more readable than others. Making it easy for others to read your code is always a good idea, and adopting a nice coding style helps tremendously for that.

For Python, PEP 8 has emerged as the style guide that most projects adhere to; it promotes a very readable and eye-pleasing coding style. Every Python developer should read it at some point; here are the most important points extracted for you:

• Use 4-space indentation, and no tabs.

  4 spaces are a good compromise between small indentation (allows greater nesting depth) and large indentation (easier to read). Tabs introduce confusion, and are best left out.

• Wrap lines so that they don’t exceed 79 characters.

  This helps users with small displays and makes it possible to have several code files side-by-side on larger displays.

• Use blank lines to separate functions and classes, and larger blocks of code inside functions.

• When possible, put comments on a line of their own.

• Use docstrings.

• Use spaces around operators and after commas, but not directly inside bracketing constructs: a = f(1, 2) + g(3, 4).

• Name your classes and functions consistently; the convention is to use CamelCase for classes and lower_case_with_underscores for functions and methods. Always use self as the name for the first method argument (see A First Look at Classes for more on classes and methods).

• Don’t use fancy encodings if your code is meant to be used in international environments. Plain ASCII works best in any case.

註解

1

Actually, call by object reference would be a better description, since if a mutable object is passed, the caller will see any changes the callee makes to it (items inserted into a list).