After this excursion into the realm of package management and package managers, let's come back to the previous topics covered in this lesson (list comprehension, web access, GUI development) and wrap up the lesson with a few practice exercises. These are meant to give you the opportunity to test how well you have understood the main concepts from this lesson and as a preparation for the homework assignment in which you are supposed to develop a small standalone and GUI-based program for a relatively simple GIS workflow using arcpy. Even if you don't manage to implement perfect solutions for all three exercises yourself, thinking about them and then carefully studying the provided solutions will be helpful, in particular since reading and understanding other people's code is an important skill and one of the main ways to become a better programmer. The solutions to the three practice exercises can be found in the following subsections.
You have a list that contains dictionaries describing spatial features, e.g. obtained from some web service. Each dictionary stores the id, latitude, and longitude of the feature, all as strings, under the respective keys "id", "lat", and "lon":
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" } ]
We want to convert this list into a list of 3-tuples instead using a list comprehension (Section 2.2). The first element of each tuple should be the id but with the fixed string "Feature " as prefix (e.g. "Feature A"). The other two elements should be the lat and lon coordinates but as floats not as strings. Here is an example of the kind of tuple we are looking for, namely the one for the first feature from the list above: ('Feature A', 23.32, -54.22). Moreover, only features with a longitude coordinate < 0 should appear in the new list. How would you achieve this task with a single list comprehension?
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)]