Python的隐藏功能

#1 楼

>>> x = 5
>>> 1 < x < 10
True
>>> 10 < x < 20 
False
>>> x < 10 < x*10 < 100
True
>>> 10 > x <= 9
True
>>> 5 == x > 4
True

#2 楼

>>> re.compile("^\[font(?:=(?P<size>[-+][0-9]{1,2}))?\](.*?)[/font]",
    re.DEBUG)
at at_beginning
literal 91
literal 102
literal 111
literal 110
literal 116
max_repeat 0 1
  subpattern None
    literal 61
    subpattern 1
      in
        literal 45
        literal 43
      max_repeat 1 2
        in
          range (48, 57)
literal 93
subpattern 2
  min_repeat 0 65535
    any None
in
  literal 47
  literal 102
  literal 111
  literal 110
  literal 116

>>> re.compile("""
 ^              # start of a line
 \[font         # the font tag
 (?:=(?P<size>  # optional [font=+size]
 [-+][0-9]{1,2} # size specification
 ))?
 \]             # end of tag
 (.*?)          # text between the tags
 \[/font\]      # end of the tag
 """, re.DEBUG|re.VERBOSE|re.DOTALL)

#3 楼

>>> a = ['a', 'b', 'c', 'd', 'e']
>>> for index, item in enumerate(a): print index, item
...
0 a
1 b
2 c
3 d
4 e
>>>

#4 楼

x=(n for n in foo if bar(n))

for n in x:

x = [n for n in foo if bar(n)]

>>> n = ((a,b) for a in range(0,2) for b in range(4,6))
>>> for i in n:
...   print i 

(0, 4)
(0, 5)
(1, 4)
(1, 5)

#5 楼

def seek_next_line(f):
    for c in iter(lambda: f.read(1),'\n'):
        pass

#6 楼

>>> def foo(x=[]):
...     x.append(1)
...     print x
... 
>>> foo()
[1]
>>> foo()
[1, 1]
>>> foo()
[1, 1, 1]

>>> def foo(x=None):
...     if x is None:
...         x = []
...     x.append(1)
...     print x
>>> foo()
[1]
>>> foo()
[1]

#7 楼

def mygen():
    """Yield 5 until something else is passed back via send()"""
    a = 5
    while True:
        f = (yield a) #yield a and possibly get f in return
        if f is not None: 
            a = f  #store the new value

>>> g = mygen()
>>> g.next()
5
>>> g.next()
5
>>> g.send(7)  #we send this back to the generator
7
>>> g.next() #now it will yield 7 until we send something else
7

#8 楼

from __future__ import braces

#9 楼

a = [1,2,3,4,5]
>>> a[::2]  # iterate over the whole list in 2-increments
[1,3,5]

>>> a[::-1]
[5,4,3,2,1]

#10 楼

>>> def print_args(function):
>>>     def wrapper(*args, **kwargs):
>>>         print 'Arguments:', args, kwargs
>>>         return function(*args, **kwargs)
>>>     return wrapper

>>> @print_args
>>> def write(text):
>>>     print text

>>> write('foo')
Arguments: ('foo',) {}
foo

#11 楼

for i in foo:
    if i == 0:
        break
else:
    print("i was never 0")

found = False
for i in foo:
    if i == 0:
        found = True
        break
if not found: 
    print("i was never 0")

#12 楼

>>> class MyDict(dict):
...  def __missing__(self, key):
...   self[key] = rv = []
...   return rv
... 
>>> m = MyDict()
>>> m["foo"].append(1)
>>> m["foo"].append(2)
>>> dict(m)
{'foo': [1, 2]}

>>> from collections import defaultdict
>>> m = defaultdict(list)
>>> m["foo"].append(1)
>>> m["foo"].append(2)
>>> dict(m)
{'foo': [1, 2]}

#13 楼

>>> a = 10
>>> b = 5
>>> a, b
(10, 5)

>>> a, b = b, a
>>> a, b
(5, 10)

#14 楼

>>> pattern = """
... ^                   # beginning of string
... M{0,4}              # thousands - 0 to 4 M's
... (CM|CD|D?C{0,3})    # hundreds - 900 (CM), 400 (CD), 0-300 (0 to 3 C's),
...                     #            or 500-800 (D, followed by 0 to 3 C's)
... (XC|XL|L?X{0,3})    # tens - 90 (XC), 40 (XL), 0-30 (0 to 3 X's),
...                     #        or 50-80 (L, followed by 0 to 3 X's)
... (IX|IV|V?I{0,3})    # ones - 9 (IX), 4 (IV), 0-3 (0 to 3 I's),
...                     #        or 5-8 (V, followed by 0 to 3 I's)
... $                   # end of string
... """
>>> re.search(pattern, 'M', re.VERBOSE)

>>> p = re.compile(r'(?P<word>\b\w+\b)')
>>> m = p.search( '(((( Lots of punctuation )))' )
>>> m.group('word')
'Lots'

>>> pattern = (
...     "^"                 # beginning of string
...     "M{0,4}"            # thousands - 0 to 4 M's
...     "(CM|CD|D?C{0,3})"  # hundreds - 900 (CM), 400 (CD), 0-300 (0 to 3 C's),
...                         #            or 500-800 (D, followed by 0 to 3 C's)
...     "(XC|XL|L?X{0,3})"  # tens - 90 (XC), 40 (XL), 0-30 (0 to 3 X's),
...                         #        or 50-80 (L, followed by 0 to 3 X's)
...     "(IX|IV|V?I{0,3})"  # ones - 9 (IX), 4 (IV), 0-3 (0 to 3 I's),
...                         #        or 5-8 (V, followed by 0 to 3 I's)
...     "$"                 # end of string
... )
>>> print pattern
"^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$"

#15 楼

def draw_point(x, y):
    # do some magic

point_foo = (3, 4)
point_bar = {'y': 3, 'x': 2}

draw_point(*point_foo)
draw_point(**point_bar)

#16 楼

#!/usr/bin/env python
# -*- coding: rot13 -*-

cevag "Uryyb fgnpxbiresybj!".rapbqr("rot13")

#17 楼

>>> NewType = type("NewType", (object,), {"x": "hello"})
>>> n = NewType()
>>> n.x
"hello"

>>> class NewType(object):
>>>     x = "hello"
>>> n = NewType()
>>> n.x
"hello"

#18 楼

from __future__ import with_statement

with open('foo.txt', 'w') as f:
    f.write('hello!')

#19 楼

#20 楼

class Property(object):
    def __init__(self, fget):
        self.fget = fget

    def __get__(self, obj, type):
        if obj is None:
            return self
        return self.fget(obj)

class MyClass(object):
    @Property
    def foo(self):
        return "Foo!"

#21 楼

x = 3 if (y == 1) else 2

x = 3 if (y == 1) else 2 if (y == -1) else 1

(func1 if y == 1 else func2)(arg1, arg2) 

x = (class1 if y == 1 else class2)(arg1, arg2)

#22 楼

def factorial(n):
    """Return the factorial of n, an exact integer >= 0.

    If the result is small enough to fit in an int, return an int.
    Else return a long.

    >>> [factorial(n) for n in range(6)]
    [1, 1, 2, 6, 24, 120]
    >>> factorial(-1)
    Traceback (most recent call last):
        ...
    ValueError: n must be >= 0

    Factorials of floats are OK, but the float must be an exact integer:
    """

    import math
    if not n >= 0:
        raise ValueError("n must be >= 0")
    if math.floor(n) != n:
        raise ValueError("n must be exact integer")
    if n+1 == n:  # catch a value like 1e300
        raise OverflowError("n too large")
    result = 1
    factor = 2
    while factor <= n:
        result *= factor
        factor += 1
    return result

def _test():
    import doctest
    doctest.testmod()    

if __name__ == "__main__":
    _test()

#23 楼

>>> print "The %(foo)s is %(bar)i." % {'foo': 'answer', 'bar':42}
The answer is 42.

>>> foo, bar = 'question', 123

>>> print "The %(foo)s is %(bar)i." % locals()
The question is 123.

>>> print("The {foo} is {bar}".format(foo='answer', bar=42))

#24 楼

#25 楼

try:
  put_4000000000_volts_through_it(parrot)
except Voom:
  print "'E's pining!"
else:
  print "This parrot is no more!"
finally:
  end_sketch()

#26 楼

# Python 2 syntax
try:
    some_operation()
except SomeError, e:
    if is_fatal(e):
        raise
    handle_nonfatal(e)

# Python 3 syntax
try:
    some_operation()
except SomeError as e:
    if is_fatal(e):
        raise
    handle_nonfatal(e)

#27 楼

import this
# btw look at this module's source :)

#28 楼

try:
    import readline
except ImportError:
    print "Unable to load readline module."
else:
    import rlcompleter
    readline.parse_and_bind("tab: complete")


>>> class myclass:
...    def function(self):
...       print "my function"
... 
>>> class_instance = myclass()
>>> class_instance.<TAB>
class_instance.__class__   class_instance.__module__
class_instance.__doc__     class_instance.function
>>> class_instance.f<TAB>unction()

#29 楼

[(i,j) for i in range(3) for j in range(i) ]    
((i,j) for i in range(4) for j in range(i) )

#30 楼

>>> a = set([1,2,3,4])
>>> b = set([3,4,5,6])
>>> a | b # Union
{1, 2, 3, 4, 5, 6}
>>> a & b # Intersection
{3, 4}
>>> a < b # Subset
False
>>> a - b # Difference
{1, 2}
>>> a ^ b # Symmetric Difference
{1, 2, 5, 6}