Lack Understanding of Multi-Variable Assignments Python

Question

I'm new with Python (with Java as a basic). I read Dive Into Python books, in the Chapter 3 I found about Multi-Variable Assignment. Maybe some of you can help me to understand what happen in this code bellow:

>>> params = {1:'a', 2:'b', 3:'c'}
>>> params.items() # To display list of tuples of the form (key, value).
[(1, 'a'), (2, 'b'), (3, 'c')]

>>> [a for b, a in params.items()] #1
['a', 'b', 'c']
>>> [a for a, a in params.items()] #2
['a', 'b', 'c']
>>> [a for a, b in params.items()] #3
[ 1 ,  2 ,  3 ]
>>> [a for b, b in params.items()] #4
[ 3 ,  3 ,  3 ]

What I understand so far is the #1 and #2 has same output, that display the values of tuple. #3 display the key of tuple, and #4 just display the last key from the list of tuples.

I don't understand the use of the variable a and variable b for every case above:

1. a for b, a ... -> display the values
2. a for a, a ... -> display the values
3. a for a, b ... -> display the keys
4. a for b, b ... -> display the last key

Can anyone elaborate the flow of the loop for every case above?

Answer 1

The list comprehension you use there roughly translate as follows:

[a for b, a in params.items()]

becomes

result = []
for item in params.items():
    b = item[0]
    a = item[1]
    result.append(a)

---

[a for a, a in params.items()]

becomes

result = []
for item in params.items():
    a = item[0]
    a = item[1] # overwrites previous value of a, hence this yields values, 
                # not keys
    result.append(a)

---

[a for a, b in params.items()]

becomes

result = []
for item in params.items():
    a = item[0]
    b = item[1]
    result.append(a)

---

[a for b, b in params.items()]

becomes

result = []
for item in params.items():
    b = item[0]
    b = item[1]
    result.append(a) # note use of a here, which was not assigned

This last one is special. It only worked because you had used the variable a in a previous statement, and the last value that had been assigned to it was 3. If you execute this statement first, you'd get an error.