Detailed description of Python's Collections built-in module

ccollections is a built-in module of python. The source code is located in Lib/collections/init.py. This module provides a universal data container.

deque containerObject

is introduced through from collections import deque. When creating a deque container object, you can set the parameter to an Iterable object (such as tuple, list, str) or maxlen=x (int type) or None for initialization.

deque container supports threadssafety. When inserting or removing elements from both ends of deque through append or pop, the time complexity is O(1) . Compared with the list object, the list also has the same api to implement the same function, but for operations on the list such as pop(0) or insert(0, x), the time complexity is O(n) .

If maxlen is not declared or maxlen=None is declared when initializing deque, the deque container can hold any number of elements. Otherwise, the deque container will be defined as a limited-length element container.

Once the number of elements in the container reaches the set maxlen, when new elements are added, the same number of elements will be excluded from the other end of the added element, thus ensuring that the elements in the current deque All are newly added elements.

deque objectfunction

append(x)

appendleft(x)

clear()

copy()

count(x): Returns the number of elements with value x in the container

extend(iterable)

extendleft(iterable)

index(x): Find the first value of x in the container Element index , if it does not exist, throw ValueError exception

insert(idx, x)

pop()

popleft()

remove(x)

reverse(): flip the elements in the container and return None

rotate(n)

deque object read-only attribute

maxlen

In addition to the above object functions, since the deque object is also an Iterable object, then len(deque);reversed(deque);copy.copy(deque);copy. Functions such as deepcopy(deque) also work. The same in operator is also used when traversing the deque operation. The slicing operation deque[-1] can also return the last element in the container. If you operate on random elements in the container, it is recommended to use list.

demo

Get the content of the line where the python string is located in the file, and the first three lines of this line

from collections import deque


def search(lines, pattern, maxlen):
    pre_lines = deque(maxlen=maxlen)
    for line in lines:
        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, end='')
            print(line)

Enter the text file content For

c#
c
c++
javascript
python
java
delphi
python
golang
perl
css
html
python

the code output is

c
c++
javascript
python

python
java
delphi
python

perl
css
html
python

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