?????3. ??????? Python ?????Python ??? ???5. ?????

??????

• 4.1 if ???

• 4.2 for ???

• 4.3 range() ????

• 4.4 break ?? continue ?????? Loops ?е? else ???

• 4.5 pass ???

• 4.6 ???庯??

• 4.7 ??????????

  • 4.7.1 ????????

  • 4.7.2 ?????????

  • 4.7.3 ????????

  • 4.7.4 Lambda ???

  • 4.7.5 ????????

 
4. ??????????????

??????????? while ???Python ???????????н?????Щ????????????????????

 
4.1 if ???

??????о??????????? if ??????磺

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

??????? 0 ????? elif ?????else ????????????“elif ” ??“ else if ”????д???????????Ч??????????????if ... elif ... elif ... ????????????????????е? switch ?? case ???

 
4.2 for ???

Python?е?for  ???????? C ?? Pascal ?????????в???? ?????????????????????????????????????Pascal??????????????????????????????????C????Python ?? for  ??????????????У??????????????е??????????????????е?????????е????????磨??а??????

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

????????????????????в??????????????????????????????????????????????????????????????????????У????磬???????????????????????????????????????????????????????????

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

 
4.3 range() ????

?????????????????У????ú???range()??????????????????????????????

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

range(10)?????????????10?????????????????????????????????????????10???б??????????????в???????Χ?е??????????????range?????????????????????????????????????????????????????????????????“????”????

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

?????????????????????????????????range()??len()??

>>> 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

 
4.4 break ?? continue ??? ???????е? else ???

break????C?е??????????????????????for??while?????

continue ??????C?н??????????????????????????ε?????

????????????else???;??????????????????б??????for????????????false??????while?????У????????break?????????2?????С?????????????????????????????????

>>> 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

 
4.5 pass ???

pass ????????????????????Щ????????????????????????????????????????磺

>>> while True:
...       pass # Busy-wait for keyboard interrupt
...

 
4.6 ???庯??

????????д??????????????и????????????????У?

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

?????def ?????????????????????????????к???????????????????????????????????????п???????????????????????????п??????????????????????????ú????? ????????????????docstring?? 

??Щ??????????????????????????????????????????????????;?????????м????????????????????????????????????

???ú??????????????????????μ?????????е??????????洢??????????????С????ò???????????????????в?????????????????????????????????????????????????????????????????????????????????????????global???????????

??????????????????????????????????????????????????????????????????????????????????????????????^4.1 ?????????????????????????????μ????????????ù????б???????

??????????????????????????????????????庯??????????????????????????????????????????????????????????????????????????á????????????????????

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

????????fib?????????????function????????????????procedure????Python??C?????????????????з????????????????????????????????????????????????????????????????????????? None ???????????????????????????????None??????????????????д???None?????????????????????????????????????

>>> print fib(0)
None

??????????????δ?????з????????????????????е?????????????????????

>>> 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 b < n:
...         result.append(b)    # see below
...         a, b = b, a+b
...     return result
... 
>>> f100 = fib2(100)    # call it
>>> f100                # write the result
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

???????????????????????Щ?μ?Python?????

• return????????з???????????????????return????None????????????????None??

• ???result.append(b)??????????result???????????method?????????????“????”??????????????????????obj.methodename???????obj??????????????????????????methodename???????????????????е???????????? ???????????岻??????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????append()??????????????壬?????????м??????????????????????????"result = result + [b]"??????Ч??????

 
4.7 ??????????

????????????????????????????????????????????????????????????????

 
4.7.1 ????????

????????????????????????????????????????????????????????????????????

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

??????????????????μ????????ask_ok('Do you really want to quit?')??????????????ask_ok('OK to overwrite the file?', 2)??

??????????????α????????????????

i = 5

def f(arg=i):
    print arg

i = 6
f()

will print 5.

??????棺????????????Ρ????????????????????????????????????????????????Щ???????磬???o????????????л??????????????

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

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

?????????

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

??????????????????????乲??????????????????????????????д??????

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

 
4.7.2 ?????????

???????????????????????????????????“keyword = value”?????磬???μ??????

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, "!"

?????????μ?????????????

parrot(1000)
parrot(action = 'VOOOOOM', voltage = 1000000)
parrot('a thousand', state = 'pushing up the daisies')
parrot('a million', 'bereft of life', 'jump')

??????????????????Ч???

parrot()                     # required argument missing?????????????
parrot(voltage=5.0, 'dead')  # non-keyword argument following keyword????????????з??????????
parrot(110, voltage=220)     # duplicate value for argument?????????????????????
parrot(actor='John Cleese')  # unknown keyword??δ???????

??????????б??е????????????????????????????????????????ж??????????????????????????????????????????????θ?????????????????????????ε??????????λ?ú?????????????????????????????????????????????????????

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

??????????? **name ??????????????????????????????????δ??????????????б??е????????????????????????????????? *name ???????????????????????????????л???????????????????????г?????????????б??е?????????*name ?????? **name ??????? ???磬?????????????????????

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 = keywords.keys()
    keys.sort()
    for kw in keys: print kw, ':', keywords[kw]

?????????????????

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

???????????????????

-- 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

???sort()??????????????????????????????????????????????????δ??????

 
4.7.3 ????????

??????????????????????ú?????????????????????Щ???????????????????????Щ????????????????????????????????????

def fprintf(file, format, *args):
    file.write(format % args)

 
4.7.4 Lambda ???

???????????????м??????????????????Lisp?г????????????Python?????lambda???????????????С?????????????????????????????????????????????“lambda a, b: a+b”?? Lambda ????????????κ???????????????????????????????????????????????????????????????????????????е?????????ɡ????????????????壬lambda?????????????Χ???????????

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

 
4.7.5 ????????

?????????????????????????

??????????????????????顣??????????????????????????????????????????????????????????????????????????????????????????????????????????д?????????????β??

????????????ж??У???????????????????????????????????????????????????????????????????????????????Ч????

Python??????????????е??????????????????????????????????????????????????????????????????????????????о????????????????????????????????????????????????????????????????????????????????????“????”?????????????????? ???ж???????????????????????????????е????????????????????????????????????????????8????????????????ò????????????????μ????????????

????????????????????????????

>>> 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.

???

... ??????^4.1

???????????????????????????????????????????????????????????????????????κθ?????????в???????????

---

?????3. ??????? Python ?????Python ??? ???5. ?????

Release 2.3, documentation updated on July 29, 2003.