在Python中解析大量数据时，如何处理索引超出范围的错误？

Question

I have a huge amount of data an a .txt file that I'm trying to parse to objects in a list using Pyhon . The data structure looks like this for most part, and when it does, the parsing is successful.

2315462;3/13/2015 8:00:00 AM;3/13/2015 1:00:00 PM
778241;1/3/2015 12:30:00 PM;1/3/2015 1:00:00 PM

As you can see, there's an id, a start time and an end time. It is parsed using this code:

my_array_with_objects = []

with open("test.txt", newline='\n') as f:
reader = csv.reader(f, delimiter=';')

for row in reader:
    my_array_with_objects.append(Employee(row[0], row[1], row[2]))

Employee being a class that looks like this:

class Employee:

def __init__(self, id, time_start, time_end):
    self.id = id
    self.time_start = time_start
    self.time_end = time_end

Occasionally though, time_end is missing from the data:

276908;1/3/20152015 8:00:00 AM

At this point the program crashes with an index out of range exception. I'm new to Python but heard there is no such thing as a null value. Then why does it crash? I assumed that it could be handled with something along the line:

if row[2] is None:
    print("error, do things to fix")

...but it doesn't trigger. How do I handle these errors? I don't want anything special to happen if the row[2] is missing. It's fine with an empty value.

Answer 1

You could add a check if len(row) < 3 as suggested by @Torxed. A better solution might be to rewrite Employee class and use the 'splat' operator to expand the row (a list). For missing values an empty string '' is used.

This also covers the cases where both start_time and end_time, or all 3 values are missing.

class Employee:
    def __init__(self, id='', start_time='', end_time=''):
        self.id = id
        self.start_time = start_time
        self.end_time = end_time

        # check values and convert to int, datetime...

for row in reader:
    my_array_with_objects.append(Employee(*row))

Answer 2

If you want to cover missing time_end, this should do the trick:

for row in reader:
    try:
        my_array_with_objects.append(Employee(row[0], row[1], row[2]))
    except IndexError:
        my_array_with_objects.append(Employee(row[0], row[1], None))

You can replace None with a default value or choose how to deal with the missing field however you want in the except block