Python's collections built-in module detailed description

The collections is a python built-in module with source code located in lib/collections/init.py, which provides a common data container.

Deque Container Object

Introduced by the From collections import deque, when you create a Deque container object, you can initialize the Iterable object (such as TUPLE,LIST,STR) or maxlen=x (int type) or None by setting parameters.

The Deque container supports thread safety, and the time complexity is O (1) when inserting or removing elements through append or pop on both ends of the deque. The list also has the same API for the same functionality as the list object, but the time complexity is O (n) for the operation of the list, such as pop (0) or insert (0, X).

If you do not declare maxlen or declare maxlen=none when initializing deque, the Deque container can hold any number of elements, otherwise the Deque container is defined as a finite-length element container.

Once the number of elements in the container reaches the set MaxLen, when a new element is added, the same number of elements is excluded at the other end of the join element, which guarantees that the elements in the current deque are all newly added elements.

Deque Object Functions

Append (x)

Appendleft (x)

Clear ()

Copy ()

Count (x): Returns the number of elements in the container that have a value of X

Extend (iterable)

Extendleft (iterable)

Index (x): The first element in the container that has a value of x is indexed, and if it does not, throws a ValueError exception

Insert (idx, x)

Pop ()

Popleft ()

Remove (x)

Reverse (): Flips the element in the container and returns None

Rotate (N)

Deque object Read-only property

MaxLen

In addition to the object functions described above, Len (deque), reversed (deque) copy.copy (deque), Copy.deepcopy (deque), and so on, because the Deque object is also a Iterable object, also The in operator is also used when traversing the deque operation, and the slice operation Deque[-1] can also return the last element in the container. It is recommended to use list if you are manipulating random elements in a container.

Demo

Gets the line of the Python string in the file, and the first three lines of this line of content

From collections import Dequedef search (lines, pattern, maxlen):    pre_lines = deque (Maxlen=maxlen) for line in    Lin ES:        If pattern in line:            yield line, pre_lines        pre_lines.append (line) if name = = ' main ': With    (open ('./ Test.txt ') as F:        for line, pre_lines in search (f, ' Python ', 3): For            pre_line in Pre_lines:                print (Pre_line, en D= ")            print (line)

Enter the contents of the text file as

C#cc++javascriptpythonjavadelphipythongolangperlcsshtmlpython

Output by Code as

Cc++javascriptpythonpythonjavadelphipythonperlcsshtmlpython