2.9.1 Lesson 2 Practice Exercise 1 Solution

features = [ { "id": "A", "lat": "23.32", "lon": "-54.22" }, 
             { "id": "B", "lat": "24.39", "lon": "53.11" }, 
             { "id": "C", "lat": "27.98", "lon": "-54.01" } ]
             
featuresAsTuples = [ ("Feature " + feat['id'], float(feat['lat']), float(feat['lon']) ) for feat in features if float(feat['lon']) < 0 ] 

print(featuresAsTuples) 

Let's look at the components of the list comprehension starting with the middle part:

for feat in features

This means we will be going through the list features using a variable feat that will be assigned one of the dictionaries from the features list. This also means that both the if-condition on the right and the expression for the 3-tuples on the left need to be based on this variable feat.

if float(feat['lon']) < 0 

Here we implement the condition that we only want 3-tuples in the new list for dictionaries that contain a lon value that is < 0. 

("Feature " + feat['id'], float(feat['lat']), float(feat['lon']) )

Finally, this is the part where we construct the 3-tuples to be placed in the new list based on the dictionaries contained in variable feat. It should be clear that this is an expression for a 3-tuple with different expressions using the values stored in the dictionary in variable feat to derive the three elements of the tuple. The output produced by this code will be:

Output:

[('Feature A', 23.32, -54.22), ('Feature C', 27.98, -54.01)]