Stuck trying to iterate through a list in Python to add extracted variables to a dictionary [duplicate]

Question

I have a list that contains data for different rivers and want to extract only the name and flow rates and insert those in a dictionary, with the name as the key and a list of the flowrates as the values. To do this I made a another list of each row that was divided better and worked from there. However, when I run the code I encounter an error on append dictionary line that simply states KeyError: 'Avon', which I don't understand so I haven't been able to find a solution.

river_info = {}

flow_info = []

river_data = [
    "Avon,27/05/2020 12:00:00 a.m.,1.634",
    "Avon,27/05/2020 1:00:00 a.m.,1.628",
    "Hakatere,27/05/2020 12:00:00 a.m.,9.813",
    "Hakatere,27/05/2020 1:00:00 a.m.,9.813",
    "Rangitata,27/05/2020 12:00:00 a.m.,50.699",
    "Rangitata,27/05/2020 1:00:00 a.m.,50.790",
    "Waimakariri,27/05/2020 12:00:00 a.m.,73.598",
    "Waimakariri,27/05/2020 1:00:00 a.m.,73.236",
]

i = 0

for i in range(len(river_data)):
    river = river_data[i].split(",")
    river_name = str(river[0])
    river_flow = float(river[-1])
    flow_info.append(river_flow)
    river_info[river_name].append(flow_info)
    i += 1

print(flow_info)

Answer 1

You will find the setdefault() function useful for this.

river_info = dict()
flow_info = list()

river_data = [
    "Avon,27/05/2020 12:00:00 a.m.,1.634",
    "Avon,27/05/2020 1:00:00 a.m.,1.628",
    "Hakatere,27/05/2020 12:00:00 a.m.,9.813",
    "Hakatere,27/05/2020 1:00:00 a.m.,9.813",
    "Rangitata,27/05/2020 12:00:00 a.m.,50.699",
    "Rangitata,27/05/2020 1:00:00 a.m.,50.790",
    "Waimakariri,27/05/2020 12:00:00 a.m.,73.598",
    "Waimakariri,27/05/2020 1:00:00 a.m.,73.236",
]

for rd in river_data:
    river, *_, fs = rd.split(',')
    flow = float(fs)
    river_info.setdefault(river, []).append(flow)
    flow_info.append(flow)

print(river_info)
print(flow_info)

Output:

{'Avon': [1.634, 1.628], 'Hakatere': [9.813, 9.813], 'Rangitata': [50.699, 50.79], 'Waimakariri': [73.598, 73.236]}
[1.634, 1.628, 9.813, 9.813, 50.699, 50.79, 73.598, 73.236]

Answer 2

You should just check if your river name (as key) is present already in the dictionary. If yes, then append flow to it's value, else add a new entry with key as river name as value as list of flow info:

river_info = {}

river_data = [
    "Avon,27/05/2020 12:00:00 a.m.,1.634",
    "Avon,27/05/2020 1:00:00 a.m.,1.628",
    "Hakatere,27/05/2020 12:00:00 a.m.,9.813",
    "Hakatere,27/05/2020 1:00:00 a.m.,9.813",
    "Rangitata,27/05/2020 12:00:00 a.m.,50.699",
    "Rangitata,27/05/2020 1:00:00 a.m.,50.790",
    "Waimakariri,27/05/2020 12:00:00 a.m.,73.598",
    "Waimakariri,27/05/2020 1:00:00 a.m.,73.236",
]


for i in range(len(river_data)):
    river = river_data[i].split(",")
    river_name = str(river[0])
    river_flow = float(river[-1])
    if river_name in river_info:
        river_info[river_name].append(river_flow)
    else:
        river_info[river_name] = [river_flow]

print(river_info)