如何读取CSV文件以仅将最新的三个值放入字典中？

Question

This code reads every row from a CSV file. The first column is made to a value in the dictionary. I want to limit the value of the keys to three (with the most recent three values.)

 for row in reader:
        key = row[0]
        if key in result:
        # if the key is in dictionary
            result[key].append(row[1])
            # add what ever is in column 2 to that key
            if len(result[key]) > 3:
            # if the result is three !!
                print ("too long")
                lastThreeValues = (result[key][-3]).copy()
                result[key].clear()
                result[key] = (lastThreeValues)

        else:
            result[key] = [row[1]]

The working code is this

reader = csv.reader(open("class1.csv"))
result = {}
for row in reader:
    key = row[0]
    if key in result:
        result[key].append(row[1])
        if len(result[key]) > 3:
            result[key] = (result[key][-3:])

Answer 1

This is much more complicated than you actually need it to be, you can just use slicing to remove the first element.

>>> a
[0, 1, 2, 3]
>>> a = a[1:]
>>> a
[1, 2, 3]

All I'm doing is telling it to set the list to equal itself from index 1 to the end. So essentially that's cutting out the first element of the list, which is your oldest value.

Though as jonrsharpe points out in the comment, using a[-3:] is neater as it literally means the last three elements in the list.

>>> a = a[-3:]
>>> a
[1, 2, 3]

Answer 2

You could use a collections.deque to greatly simplify the creation of the results dictionary. The only complication is that you might would need to convert all its values into list s afterwards, but it can be done with one line of code.

Here's what I mean:

from collections import deque
MAXLEN = 3

for row in reader:
    key = row[0]
    if key in result:
        result[key].append(row[1])
    else:
        result[key] = deque([row[1]], MAXLEN)

# convert result values into lists
result = {k: list(v) for k, v in result.iteritems()}