如何排序python命令的输出？

Question

Python beginner here. Let's say that I have something at the end of my script that queries info from a system and dumps it into this format:

print my_list_of_food(blah)

and it outputs a list like:

('Apples', 4, 4792320)
('Oranges', 2, 2777088)
('Pickles', 3, 4485120)
('Pumpkins', 1, 5074944)
('more stuff', 4545, 345345)

How do I then sort that output based on the 2nd field so that it would look like this:

('Pumpkins', 1, 5074944)
('Oranges', 2, 2777088)
('Pickles', 3, 4485120)
('Apples', 4, 4792320)

Other than importing a bash command to cut -d "," -f 2 | head 4 cut -d "," -f 2 | head 4 I'd rather use python. I know I can use sorted or sort to sort an existing tuple or dictionary but I'm not sure of how to sort the output of a print. I've done some research and everything points to implementing sort into my script instead of sorting a print output but who knows, maybe someone has a solution. Thanks.

UPDATE:

Thanks everyone. I've tried to make all of the solutions work but I keep getting this error:

File "test.py", line 18, in <lambda>
    print sorted(my_list_of_food(blah), key=lambda x: x[1])
TypeError: 'int' object is unsubscriptable

File "test.py", line 18, in <lambda>
    print(sorted(my_list_of_food(blah), key=lambda k: k[1]))
TypeError: 'int' object is unsubscriptable

I tried to include this at the beginning of the script:

from __future__ import print_function 

but no luck.

Answer 1

You can use the key argument to the sort. In your case,

print(sorted(list_of_food, key=lambda k:k[1]))

will do the trick. The key function should return an integer, usually.

Answer 2

If it prints out that way, than either my_list_of_food(blah) returns a string, or it returns a class instance that has a __repr__ method that returns that string... Your best bet is to get data in the actual list format before it becomes a string.

If it returns a class instance, get the list and sort on it using key... Otherwise you need to parse the text, so I'll address that part only:

# this is assuming the structure is consistent
# function to parse a line of the form "('{text}', {int}, {int})" into tuple members using regex
import re
re_line = re.compile(r"\('(?P<name>\w*)',\s?(?P<int1>\d+)\s?,\s?(?P<int2>\d+)\)")
def ParseLine(line):
    m = re_line.match(line)
    if m:
        return (m.group('name'), int(m.group('int1')), int(m.group('int2')))
    # otherwise return a tuple of None objects
    else:
        return (None, None, None)

# final sorted output (as tuples of (str, int, int) )
sorted( 
    [ParseLine(line) for line in my_list_of_food(blah).splitlines()],
    key = lambda tup: tup[1]
)

Answer 3

You can't sort after outputting to stdout . Well, you shouldn't, since it's heavily complicating a simple task. Instead, you sort the value and print the result:

print sorted(my_list_of_food(blah), key=lambda x: x[1])

The part where you were having difficulties is sorting by the second field; that's why that key argument is there - to override the default ordering (which would be the first item of the tuple). To learn more about key functions, check this section on the python Sorting HOW TO

If you're still wondering, technically, you could sort the stdout output if you replaced sys.stdout with a wrapper object that buffered the output then sorted and printed it (to the real stdout) periodically, but it's pointless and hard to get right, IMHO.

Answer 4

OK: If you really have to do it that way: capture the output of the function in a StringIO object and sort the lines read back from that object.

import sys
try:
    from StringIO import StringIO
except:
    from io import StringIO

a = [ ('Apples', 4, 4792320), 
    ('Oranges', 2, 2777088), 
    ('Pickles', 3, 4485120), 
    ('Pumpkins', 1, 5074944), 
    ('more stuff', 4545, 345345) ]

[Step 1]

out = StringIO()    # create a StringIO object
sys.stdout = out    # and redirect stdout to it
for item in a : print (item)        # print output
sys.stdout = sys.__stdout__     # redirect stdout back to normal stdout

out.seek(0)     # rewind StringIO object
s = out.readlines() # and read the lines.

[Step 2: define a key function to split the strings at comma,

and compare the 2nd field numerically. ]

def sortkey(a):
   return int(a.split(',')[1].strip())

[Step 3: sort and print]

s.sort(key=sortkey)
for line in s: print (line)