Python Sort - Alexander Lee

1) Sort a List (In-Place)#

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a = [3, 1, 2]
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a.sort()
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print(a)  # [1, 2, 3]

2) Return a New Sorted List (Do Not Modify the Original)#

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a = [3, 1, 2]
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b = sorted(a)
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print(b)  # [1, 2, 3]
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print(a)  # [3, 1, 2]

3) Sort in Descending Order#

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a.sort(reverse=True)
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# or
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b = sorted(a, reverse=True)

4) Sort by Key (Common for Tuples / Objects)#

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arr = [(1, 5), (2, 3), (3, 4)]
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arr.sort(key=lambda x: x[1])
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print(arr)  # sort by the second element

5) Sort by multiple key and value#

1.Ascending, Ascending#

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arr.sort(key=lambda x: (x[0], x[1]))

Example:

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arr = [(2, 0), (1, 4), (2, 1), (1, 3)]
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arr.sort(key=lambda x: (x[0], x[1]))
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print(arr)
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# [(1, 3), (1, 4), (2, 0), (2, 1)]

Meaning:

sort by x[0] first
if x[0] is equal, sort by x[1]

2.Ascending + Descending#

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arr.sort(key=lambda x: (x[0], -x[1]))

Example:

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# Ascending + Descending (most common in contests)
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arr = [(2, 0), (1, 4), (2, 1), (1, 3)]
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arr.sort(key=lambda x: (x[0], -x[1]))
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print(arr)
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# [(1, 4), (1, 3), (2, 1), (2, 0)]

3.All descending (two ways)#

Method A: reverse=True (global reverse) reverse=True#

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arr.sort(key=lambda x: (x[0], x[1]), reverse=True)

⚠️ This reverses the whole result, not “first asc, second desc”.

Method B: negate each key (more flexible)#

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arr.sort(key=lambda x: (-x[0], -x[1]))

4.key&value sorting for strings#

Sort by length descending, then lexicographically ascending

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words.sort(key=lambda s: (-len(s), s))

Example:

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words = ["apple", "bat", "banana", "app"]
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words.sort(key=lambda s: (-len(s), s))
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print(words)
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# ['banana', 'apple', 'app', 'bat']

5.Sort a dictionary / Counter by value, then key#

Example: sort by frequency descending, then number ascending

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from collections import Counter
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# Sort a dictionary / Counter by value, then key
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nums = [1,1,1,2,2,3,3,4]
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cnt = Counter(nums)
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print(cnt)
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res = sorted(cnt.items(), key=lambda x: (-x[1], x[0]))
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print(res)
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# Counter({1: 3, 2: 2, 3: 2, 4: 1})
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# [(1, 3), (2, 2), (3, 2), (4, 1)]
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nums = {4:1, 1:3, 2:2, 3:2}
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print(nums)
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res = sorted(nums.items(), key=lambda x: (-x[1], x[0]))
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print(res)
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# [(1, 3), (2, 2), (3, 2), (4, 1)]