1.2.3. 控制流

Controls the order in which the code is executed.

1.2.3.1. if/elif/else

>>> if 2**2 == 4:
...     print('Obvious!')
...
Obvious!

Blocks are delimited by indentation

在Python解释器中键入以下行，并小心保持缩进深度。Ipython shell在冒号:符号后自动增加缩进深度；要减小缩进深度，使用Backspace键向左移动四个空格。Press the Enter key twice to leave the logical block.

>>> a = 10

>>> if a == 1:
...     print(1)
... elif a == 2:
...     print(2)
... else:
...     print('A lot')
A lot

缩进在脚本中也是强制性的。作为练习，在脚本condition.py中重新键入相同缩进的上面的行，并在Ipython中使用run condition.py执行这个脚本。

1.2.3.2. for/range

Iterating with an index:

>>> for i in range(4):
...     print(i)
0
1
2
3

但是最常见的，在值上迭代更具可读性：

>>> for word in ('cool', 'powerful', 'readable'):
...     print('Python is %s' % word)
Python is cool
Python is powerful
Python is readable

1.2.3.3. while/break/continue

Typical C-style while loop (Mandelbrot problem):

>>> z = 1 + 1j
>>> while abs(z) < 100:
...     z = z**2 + 1
>>> z
(-134+352j)

More advanced features

break for/while循环：

>>> z = 1 + 1j

>>> while abs(z) < 100:
...     if z.imag == 0:
...         break
...     z = z**2 + 1

continue循环的下一次迭代:

>>> a = [1, 0, 2, 4]
>>> for element in a:
...     if element == 0:
...         continue
...     print(1. / element)
1.0
0.5
0.25

1.2.3.4. 条件表达式

if <OBJECT>:

以下值为False：

• 任何等于零（0、0.0、0+0j）的数字，
• 空容器（list、tuple、set、dictionary，...）
• False、None

以下值为True：

• 除了为Flase的所有其它的值

a == b:

测试相等性：

>>> 1 == 1.
True

a is b:

Tests identity: both sides are the same object:

>>> 1 is 1.
False

>>> a = 1
>>> b = 1
>>> a is b
True

a in b:

For any collection b: b contains a

>>> b = [1, 2, 3]
>>> 2 in b
True
>>> 5 in b
False

If b is a dictionary, this tests that a is a key of b.

1.2.3.5. 高级迭代

1.2.3.5.1. 迭代任何序列

你可以遍历任何序列（字符串、列表、字典中的键、文件中的行，...）：

>>> vowels = 'aeiouy'

>>> for i in 'powerful':
...     if i in vowels:
...         print(i)
o
e
u

>>> message = "Hello how are you?"
>>> message.split() # returns a list
['Hello', 'how', 'are', 'you?']
>>> for word in message.split():
...     print(word)
...
Hello
how
are
you?

Few languages (in particular, languages for scientific computing) allow to loop over anything but integers/indices. 使用Python，可以完全只循环感兴趣的对象，而不必麻烦你经常不关心的索引。This feature can often be used to make code more readable.

Warning

修改你正在迭代的序列是不安全的。

1.2.3.5.2. 跟踪计数

常见任务是在序列上进行迭代，同时跟踪元素编号。

• 可以使用while循环与计数器，如上。Or a for loop:

  >>> words = ('cool', 'powerful', 'readable')
  >>> for i in range(0, len(words)):
  ...     print((i, words[i]))
  (0, 'cool')
  (1, 'powerful')
  (2, 'readable')
  

• But, Python provides a built-in function - enumerate - for this:

  >>> for index, item in enumerate(words):
  ...     print((index, item))
  (0, 'cool')
  (1, 'powerful')
  (2, 'readable')
  

1.2.3.5.3. 在字典上循环

使用items：

>>> d = {'a': 1, 'b':1.2, 'c':1j}

>>> for key, val in sorted(d.items()):
...     print('Key: %s has value: %s' % (key, val))
Key: a has value: 1
Key: b has value: 1.2
Key: c has value: 1j

Note

字典的顺序是随机的，因此我们使用sorted()来排序键。

1.2.3.6. 列表推导式

>>> [i**2 for i in range(4)]
[0, 1, 4, 9]

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练习

Compute the decimals of Pi using the Wallis formula: