Menu
I'm trying to unpack a complex dictionary and I'm getting a NameError in a list comprehension expression using multiple loops:
a={
1: [{'n': 1}, {'n': 2}],
2: [{'n': 3}, {'n': 4}],
3: [{'n': 5}],
}
good = [1,2]
print [r['n'] for r in a[g] for g in good]
# NameError: name 'g' is not defined
801
From loops to comprehensions Every list comprehension can be rewritten as a for loop but not every for loop can be rewritten as a list comprehension. The key to understanding when to use list comprehensions is to practice identifying problems that smell like list comprehensions.
List comprehensions are also more declarative than loops, which means they're easier to read and understand. Loops require you to focus on how the list is created. You have to manually create an empty list, loop over the elements, and add each of them to the end of the list.
The for loop is a common way to iterate through a list. List comprehension, on the other hand, is a more efficient way to iterate through a list because it requires fewer lines of code. List comprehension requires less computation power than a for loop because it takes up less space and code.
You have the order of your loops mixed up; they are considered nested from left to right, so for r in a[g] is the outer loop and executed first. Swap out the loops:
print [r['n'] for g in good for r in a[g]]
Now g is defined for the next loop, for r in a[g], and the expression no longer raises an exception:
>>> a={
... 1: [{'n': 1}, {'n': 2}],
... 2: [{'n': 3}, {'n': 4}],
... 3: [{'n': 5}],
... }
>>> good = [1,2]
>>> [r['n'] for g in good for r in a[g]]
[1, 2, 3, 4]
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
