Advanced features of Python 2: List derivation and generator

I. List-derived

1. The list derivation is a python-style notation. This is not only efficient, but also shorter.

 in [all]: [ '  i:el   ,  " i:el   ", "  i: El  ] 

Enumerate is a built-in function that allows the list to have "subscript" properties.
If you do not use a list deduction, the above example needs to be written

In []: LST = [" One"," Both","three"]in []: i =0In [26]: forEinchLST: ....: Lst[i]='%d:%s'%(I,lst[i]) .....: I+=1... ..: In [27]: lstout[27]: ['0:one','1:two','2:three']

Two. Generator (generator)

1. When to use the generator

In fact, you do not need a generator, only if it is required because of performance constraints, for example, you need to read a 10g txt file with Python, if you load the 10g file into memory processing (. Read () method), the memory must overflow. If you use generators here, you can cross-read and process, such as using (. ReadLine and. ReadLines), which can be processed at the same time as the loop reads, saving a lot of memory space.

(PS: If you write a read-write function to encapsulate to others to use, then consider the file capacity issues, you should consider using the generator.) ）

2. Two ways to generate generators

2.1 The first one is relatively simple, the list derivation of the [] renamed () is OK

 for  in range (10))

2.2 The second option is to add the yield keyword to the function. Yield and return are a bit similar and can all be used to return a value, the difference is that yield encounters next () to return, and executes again from the last yield statement returned.

3. How to tell if a function is a generator

The way to judge the generator is to look at its properties

in [+]: Dir (g) out[]: ['__class__'__next__  ' __repr__ ' __setattr__  ',]

Here you can see that G has a __next__ magic method, which is a unique property of the generator, the following two ways to call can be

in []: G.__next__(out[): 0In []: G.__next__() out[]: 1 in[ []: Next (g) out[44]: 4

Advanced features of Python 2: List derivation and generator