Python 3.3 Tutorial Notes - 2：Control Flow

1. if, else, elif

1. Iterating over a sequence does not implicitly make a copy.

2. "for" in Python is "foreach" in C#

3. range(n) means n numbers: 0..n-1, good for for-loop

4. range(n) is an object which returns the successive items of the desired sequence when you iterate over it, but it doesn’t really make the list, thus saving space.

5. The function list() is another; it creates lists from iterables: list(range(5))

6. for-else: if not break/continue in the for loop, "else" part will be executed. -- like the try-finally in C#

1. def: doc string is in the function, not above the function. The first line in func, within """..."""

2. If no return in def, the value of the function call is "None"

3. Support default value in the function parameter

     3.1. The default values are evaluated at the point of function definition in the defining scope

     i = 5

     def f(arg=i):

          print(arg)

     i = 6

     f()

     will print 5.

3.2. 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. (the default value is a "static" one in heap)

     def f(a, L=[]):

          L.append(a)

          return L

     print(f(1))

     print(f(2))

     print(f(3))

     -------------

     output:

     [1]

     [1, 2]

     [1, 2, 3]

     ===========

     to avoid above, use below 

     def f(a, L=None):

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

4. *name and **name

     4.1. When a final formal parameter of the form **name is present, it receives a dictionary (see typesmapping) containing all keyword arguments except for those corresponding to a formal parameter.

     4.2. a formal parameter of the form *name which receives a tuple containing the positional arguments beyond the formal parameter list. 

     4.3. *name must occur before **name.

1. "in" keyword. This tests whether or not a sequence contains a certain value: if ok in (’y’, ’ye’, ’yes’):

1. Dictionary could be used as arguments set to pass in a function: <parameter name>:<value> pairs.

 

1. Lambda expression is supported.

2. Like the nested func, lambda func can reference variables from the containing scope.

1. Print(foo.__doc__) will print out the func's documentation string.