在 Python 中理解 [foo for bar, foo in arr]

Question

I have a list of tuples, such as [(4, 'c'), (5, 'o'), (6, 'd')] . I am trying to get better at extracting only the data I need. I can do it now with for loops or unpacking with zip but Im looking for other methods to expand my knowledge.

I have seen the code below used in several places which extracts the second index of the tuple

letters = [ foo for bar, foo in arr ] 

How does this work? and if I wanted to extract the first index, is that possible?

Answer 1

The [.. for .. in ..] syntax denotes a comprehension expression , specifically one for creating a list as indicated by to the enclosing [ ] . Comprehensions generally work similar to for statements in that they allow to iterate over something, assign or unpack each element, and then operate on each element.

[ foo for bar, foo in arr ] 
#                     ^ iterable providing elements
#         ^ assignment of each element to name(s)
# ^ operation for each element

Of note is that the assignment in the middle has the same power as using = for assignment:

• a single name target is assigned the entire value
  a = (1, 2) or for a in [(1, 2), (3, 4)]
• multiple name targets are assigned the parts of the value
  a, b = (1, 2) or for a, b in [(1, 2), (3, 4)]

This makes comprehensions convenient to unpack elements and operate on each part individually – for example to combine them or to discard one.

Using proper names for the parts of each element makes it more obvious what is going on, and how to change the result:

>>> # code of the question, using proper names
>>> pairs = [(4, 'c'), (5, 'o'), (6, 'd')]
>>> [letter for number, letter in pairs]
['c', 'o', 'd']
>>> # desired result of the question
>>> [number for number, letter in pairs]
[4, 5, 6]

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Comprehensions, especially for lists, are a feature found in many programming languages. They are derived from mathematical "set builder notation" that some people may be familiar with. For example, the mathematical expression "set of numbers in arr"

{ number | (number, letter) ∈ arr}

directly translates to a set comprehension:

#      { number | (number, letter) ∈ arr}
# 
nums = {number for number, letter in arr}

Answer 2

You could extract the first index in the same fashion:

letters_first = [bar for bar, foo in arr]

This shorthand technique is called "list comprehension" in Python. The general syntax is

[expression for variable in list] 

Answer 3

arr = [(4, 'c'), (5, 'o'), (6, 'd')]
letters = [ bar for bar, foo in arr ] #  it will extract the first index

Answer 4

This is called list comprehension. It creates a list with foo from bar and foo from arr.