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Jan 17th, 2007

Can list comprehension do this?

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This is part of a short script I wrote to find out whether students had submitted any corrections to work in December or January.

Python Syntax (Toggle Plain Text)
import os, os.path
 
months = ['Dec', 'Jan']  # actually a raw_input()
 
files = get_all_files(root_dir)  # actually a call to os.path.walk()
 
dates = []
 
for i in files:
    for j in months:
         if j in i:  # filenames have date-stamp on them
             dates.append(i)
             break
import os, os.path

months = ['Dec', 'Jan']  # actually a raw_input()

files = get_all_files(root_dir)  # actually a call to os.path.walk()

dates = []

for i in files:
    for j in months:
         if j in i:  # filenames have date-stamp on them
             dates.append(i)
             break

In other words, dates ends up with all of the files whose names contain any of the items in months.

Can this be done with list comprehension?

Doesn't work:

dates = [i for i in files if j in i for j in months]

Jeff

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jrcagle

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Jan 17th, 2007

Re: Can list comprehension do this?

If you want to find file date,
stats = os.stat( filename )
print time.ctime( stats[8] )
prints 'Tue Dec 26 14:28:16 2006'
string.split, etc. yields the month, although I think you should also take the year into account.

Last edited by woooee; Jan 17th, 2007 at 1:20 am.

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Jan 17th, 2007

Re: Can list comprehension do this?

can you try :

Python Syntax (Toggle Plain Text)
dates = [i for i in files for j in months if j in i  ]
dates = [i for i in files for j in months if j in i  ]

ghostdog74

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Jan 17th, 2007

Re: Can list comprehension do this?

Looks like Ghostdog is right ...

Python Syntax (Toggle Plain Text)
# test sample from user input
months = ['Dec', 'Jan']
# test sample from os.path.walk()
files = ['FebWork1,dat', 'JanWork3.dat', 'DecWork7.dat']  # test
dates = []
for i in files:
    for j in months:
         if j in i:
             dates.append(i)
             break
print dates
# ... or list comprehension ...
dates2 = [i for i in files for j in months if j in i  ]
print dates2
"""
my output -->
['JanWork3.dat', 'DecWork7.dat']
['JanWork3.dat', 'DecWork7.dat']
"""
# test sample from user input
months = ['Dec', 'Jan']
# test sample from os.path.walk()
files = ['FebWork1,dat', 'JanWork3.dat', 'DecWork7.dat']  # test
dates = []
for i in files:
    for j in months:
         if j in i:
             dates.append(i)
             break
print dates
# ... or list comprehension ...
dates2 = [i for i in files for j in months if j in i  ]
print dates2
"""
my output -->
['JanWork3.dat', 'DecWork7.dat']
['JanWork3.dat', 'DecWork7.dat']
"""

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Jan 17th, 2007

Re: Can list comprehension do this?

Ah ... it seems obvious now. :lol:

Thanks,
Jeff

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Jan 21st, 2007

Re: Can list comprehension do this?

Click to Expand / Collapse Quote originally posted by vegaseat ...

Looks like Ghostdog is right ...
Python Syntax (Toggle Plain Text)
# test sample from user input
months = ['Dec', 'Jan']
# test sample from os.path.walk()
files = ['FebWork1,dat', 'JanWork3.dat', 'DecWork7.dat']  # test
dates = []
for i in files:
    for j in months:
         if j in i:
             dates.append(i)
             break
print dates
# ... or list comprehension ...
dates2 = [i for i in files for j in months if j in i  ]
print dates2
"""
my output -->
['JanWork3.dat', 'DecWork7.dat']
['JanWork3.dat', 'DecWork7.dat']
"""
# test sample from user input months = ['Dec', 'Jan'] # test sample from os.path.walk() files = ['FebWork1,dat', 'JanWork3.dat', 'DecWork7.dat'] # test dates = [] for i in files: for j in months: if j in i: dates.append(i) break print dates # ... or list comprehension ... dates2 = [i for i in files for j in months if j in i ] print dates2 """ my output --> ['JanWork3.dat', 'DecWork7.dat'] ['JanWork3.dat', 'DecWork7.dat'] """

I sort of can see how list comprehension evolves from the individual for loops. Is there anything list comprehension can do that one could not write out with individual for loops?

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Jan 21st, 2007

Re: Can list comprehension do this?

Click to Expand / Collapse Quote originally posted by sneekula ...

Is there anything list comprehension can do that one could not write out with individual for loops?

Not in principle. However, in practice, list comprehension makes the code much more comprehensible. So: list comprehension does better code maintenance.

Mathematically,

Python Syntax (Toggle Plain Text)
mylist = [x for x in S if C]
mylist = [x for x in S if C]

is equivalent to

Python Syntax (Toggle Plain Text)
mylist = []
for x in S:
    if C:
       mylist.append(x)
mylist = []
for x in S:
    if C:
       mylist.append(x)

so it must be the case that for loops can do anything list comprehensions can do.

But the expressive power ... wow!

Python Syntax (Toggle Plain Text)
average_salary = sum([p.salary for p in personnel])/len(personnel)
average_salary = sum([p.salary for p in personnel])/len(personnel)

is exactly what I'm thinking, while

Python Syntax (Toggle Plain Text)
total = 0
for p in personnel:
   total += p.salary
average_salary = total / len(personnel)
total = 0
for p in personnel:
   total += p.salary
average_salary = total / len(personnel)

was only what I was trained to think over time.

Jeff

* Error-checking not included. Salary information void where prohibited by law. p.salary may be negative or complex in some states.

Last edited by jrcagle; Jan 21st, 2007 at 3:19 pm.

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