How to obtain the composition of sub-region elements from a two-dimensional list in Python

This article mainly introduces how to obtain the composition of sub-area elements from a two-dimensional list in Python. The article gives a detailed introduction and sample code. I believe it has certain reference value for everyone's understanding and learning. Friends in need, please come and take a look below.

Anyone who has used NumPY should know that the area slicing function can be conveniently used in a two-dimensional array, as shown below:

This function is not supported in List of the Python standard library. In List, slicing operations can only be performed in one dimension:

But sometimes I just want to use this function, but don’t want to introduce NumPY. In fact, I can also implement it in Python at this time. At this time, just implement the __getitem__ special method in a class:

class Array:
 """实现__getitem__，支持序列获取元素、Slice等特性"""

 def __init__(self, lst):
  self.__coll = lst

 def __repr__(self):
  """显示列表"""

  return '{!r}'.format(self.__coll)

 def __getitem__(self, key):
  """获取元素"""
  slice1, slice2 = key
  row1 = slice1.start
  row2 = slice1.stop
  col1 = slice2.start
  col2 = slice2.stop
  return [self.__coll[r][col1:col2] for r in range(row1, row2)]

Try it:

a = Array([['a', 'b', 'c', 'd'],
   ['e', 'f', 'g', 'h'],
   ['i', 'j', 'k', 'l'],
   ['m', 'n', 'o', 'p'],
   ['q', 'r', 's', 't'],
   ['u', 'v', 'w', 'x']])

print(a[1:5, 1:3])

Official document’s explanation of __getitem__:

In short, it is mainly used to obtain the value of self[key].

In order to highlight the solution to the problem, I only list the key codes, exception judgment, boundary checking, conditional restrictions, and even some other special methodssuch as __setitem__, __delitem__# Codes such as ## and __len__ need to be added according to the actual situation.

Of course, there are other ways to handle it, such as the code shown below, but different methods undoubtedly give me a variety of options in various scenarios.

a = [['a', 'b', 'c', 'd'],
  ['e', 'f', 'g', 'h'],
  ['i', 'j', 'k', 'l'],
  ['m', 'n', 'o', 'p'],
  ['q', 'r', 's', 't'],
  ['u', 'v', 'w', 'x']]

sl = lambda row1, row2, col1, col2, lst: \
  [lst[r][col1:col2] for r in range(row1, row2)]

sl(1, 5, 1, 3, a)

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