在Python中使用过滤器功能

Question

I am trying to use Python's built-in filter function in order to extract data from certain columns in a CSV. Is this a good use of the filter function? Would I have to define the data in these columns first, or would Python somehow already know which columns contain what data?

Answer 1

Since python boasted "batteries included", for most the everyday situations, someone might already provided a solution. CSV is one of them, there is built-in csv module

Also tablib is a very good 3rd-party module especially you're dealing with non-ascii data.

For the behaviour you described in the comment, this will do:

import csv
with open('some.csv', 'rb') as f:
   reader = csv.reader(f)
   for row in reader:
      row.pop(1)
      print ", ".join(row)

Answer 2

The filter function is intended to select from a list (or in general, any iterable) those elements which satisfy a certain condition. It's not really intended for index-based selection. So although you could use it to pick out specified columns of a CSV file, I wouldn't recommend it. Instead you should probably use something like this:

with open(filename, 'rb') as f:
    for record in csv.reader(f):
        do_something_with(record[0], record[2])

Depending on what exactly you are doing with the records, it may be better to create an iterator over the columns of interest:

with open(filename, 'rb') as f:
    the_iterator = ((record[0], record[2]) for record in csv.reader(f))
    # do something with the iterator

or, if you need non-sequential processing, perhaps a list:

with open(filename, 'rb') as f:
    the_list = [(record[0], record[2]) for record in csv.reader(f)]
    # do something with the list

I'm not sure what you mean by defining the data in the columns. The data are defined by the CSV file.

---

By comparison, here's a case in which you would want to use filter : suppose your CSV file contains numeric data, and you need to build a list of the records in which the numbers are in strictly increasing order within the row. You could write a function to determine whether a list of numbers is in strictly increasing order:

def strictly_increasing(fields):
    return all(int(i) < int(j) for i,j in pairwise(fields))

(see the itertools documentation for a definition of pairwise ). Then you can use this as the condition in filter :

with open(filename, 'rb') as f:
    the_list = filter(strictly_increasing, csv.reader(f))
    # do something with the list

Of course, the same thing could, and usually would, be implemented as a list comprehension:

with open(filename, 'rb') as f:
    the_list = [record for record in csv.reader(f) if strictly_increasing(record)]
    # do something with the list

so there's little reason to use filter in practice.