Python try-except multiple values in loop and skip current iteration if any exceptions raised

Question

I'm converting values to float from multiple columns in a dataset concurrently in a loop, where I try to skip bad rows with a ValueError exception

My data has been loaded with a structure that enables me to loop over it as such

A_floated = []
B_floated = []

for i in range(len(data)):

    A = data[i]["column_A"]
    B = data[i]["column_B"]

    try:
        A = float(A)
        A_floated.append(A)

        B = float(B)
        B_floated.append(B)

    except(ValueError):
        continue

I want the exception to encompass both A and B, so that if any of them contains a bad row, all of them will be skipped for this i, so that len(A_floated) == len(B_floated). Right now, I can only figure out how to skip bad rows on an individual-column basis, but then I end up with floated columns of different lengths.

How can this be done?

Sergii Shcherbak · Accepted Answer · 2017-10-31 13:10:56Z

3

If I understand you correctly, you want to gather 'good' (floated) rows in data structures A_floated and B_floated respectively during iteration through data. If so, try putting the former data structures outside the loop, and check whether values can be converted into floats before putting them into lists. Otherwise, value A may be convertible while value B not, which would result in different lengths of A_floated and B_floated. To sum it up, try the following code:

A_floated = []
B_floated = []

for i in range(len(data)):

    A = data[i]["column_A"]
    B = data[i]["column_B"]

    try:
        # "floatability" check.
        A = float(A)
        B = float(B)

        # If both values can be converted into a float,
        # put them into the respective lists.
        A_floated.append(A)
        B_floated.append(B)

    except(ValueError):
        continue

answered Oct 31, 2017 at 13:10

Sergii Shcherbak

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2 Comments

komodovaran_ Over a year ago

Oops, I forgot to put the empty lists outside of the loop while I wrote the example! Sorry for the blunder.

komodovaran_ Over a year ago

Also, I didn't know that both of the float checks needed to be called first before appending, and that it would be different from a try block with a potential error somewhere inside. Thanks.

Dimitris Fasarakis Hilliard · Accepted Answer · 2017-10-31 12:55:01Z

1

Use can use the else clause which runs if no exception occurs. try and convert, don't do any appending there and, if no errors occur, you append in the else clause:

try:
    A = float(A)
    B = float(B)
except ValueError:
    continue
else:
    A_floated.append(A)
    B_floated.append(B)

I removed the parentheses around the ValueError since they aren't necessary.

answered Oct 31, 2017 at 12:55

Dimitris Fasarakis Hilliard

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