reverse() doesn't reverse sort correctly

[billje]

BPO	14542
Nosy	@terryjreedy, @mdickinson, @ericvsmith
Files	EX38.PY: Idle file

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2012-04-10.21:45:47.995>
created_at = <Date 2012-04-10.21:22:28.354>
labels = ['expert-IDLE', 'invalid', 'OS-windows', 'performance']
title = "reverse() doesn't reverse sort correctly"
updated_at = <Date 2012-04-13.17:32:46.092>
user = 'https://bugs.python.org/billje'

bugs.python.org fields:

activity = <Date 2012-04-13.17:32:46.092>
actor = 'terry.reedy'
assignee = 'none'
closed = True
closed_date = <Date 2012-04-10.21:45:47.995>
closer = 'eric.smith'
components = ['Demos and Tools', 'IDLE', 'Windows']
creation = <Date 2012-04-10.21:22:28.354>
creator = 'billje'
dependencies = []
files = ['25172']
hgrepos = []
issue_num = 14542
keywords = []
message_count = 7.0
messages = ['157988', '157990', '158026', '158029', '158031', '158041', '158214']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'mark.dickinson', 'eric.smith', 'billje']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue14542'
versions = ['Python 2.7']

[billje]

reverse() doesn't reverse sort correctly in Python 25 and 27. sort() works correctly, but reverse doesn't. The following uses reverse(). Incorrect. Even though the date comes first, reverse() sorts on the file name, "B57IBMCD_T.....zip"

2012-04-05 B57IBMCD_T7.2.3.1.zip
2012-03-23 B57IBMCD_T7.2.2.3.zip
2012-02-22 B57IBMCD_T7.2.2.2.zip
2012-02-13 B57IBMCD_T7.2.2.1.zip
2012-01-23 B57IBMCD_T7.2.1.1.zip
2012-03-28 B57IBMCD_T7.0.4.6.zip
2012-03-08 B57IBMCD_T7.0.4.5.zip
2012-03-03 B57IBMCD_T7.0.4.4.zip
2012-02-29 B57IBMCD_T7.0.4.3.zip
2012-01-06 B57IBMCD_T7.0.4.2.zip
2011-12-08 B57IBMCD_T7.0.4.1.zip
2012-01-06 B57IBMCD_T7.0.3.1.zip
2012-01-06 B57IBMCD_T7.0.2.2.zip
2012-01-06 B57IBMCD_T7.0.2.1.zip
2012-01-06 B57IBMCD_T6.2.4.9.zip
2011-12-07 B57IBMCD_T6.2.4.9-r1.zip
2011-09-13 B57IBMCD_T6.2.4.8.zip
2012-03-28 B57IBMCD_T6.2.4.12.zip
2012-02-15 B57IBMCD_T6.2.4.11.zip
2011-12-06 B57IBMCD_T6.2.4.10.zip

[ericvsmith]

list.reverse() does not reverse a list, it reverses its current values.

>>> help([].reverse)
Help on built-in function reverse:

reverse(...)
    L.reverse() -- reverse *IN PLACE*

>>

[billje]

Eric.....
Thanks for answering, but I don't understand the difference between "reversing a list" and "reversing it's current values". If I have a list containing elements: A, B, C, D and reverse the list's current values, I get: D, C, B, A, which is the same as reversing the list. Please explain in more detail. Perhaps you can recommend good reference material. Since sort() sorts a list ascending (and works correctly), shouldn't reverse() sort the list descending? Am I using the wrong function?  I have attached my program:
Bill.....

#EX38.PY Get listing of all files in a directory
#http://stackoverflow.com/questions/237079/how-to-get-file-creation-modification-date-times-in-python
# Answer 37
import os, datetime
topDir = "C:/BrcmCDs"
dateFirst = True # put dates before file names
# DateFirst = False # put file names before dates

def modDate(fileN): # returns a 10-place date string
    t = os.path.getmtime(directory+"/"+fileN)
    return str(datetime.datetime.fromtimestamp(t))[0:10]
    
for directory, dirnames, filenames in os.walk(topDir):
    outList = []
    print "Directory: ", directory
    print

    for fileA in filenames:
        if (fileA[0:10] == "B57IBMCD_T" and
            fileA[-4:] == ".zip"):

            dateString = modDate(fileA)
            if dateFirst:
               fileString = dateString + " " + fileA
            else:
               fileString = fileA + " " + dateString
           
            outList.append(fileString)
           
outList.sort() # sort the file list, ascending
# outList.reverse() # reverse the file list, desending
       
print outList #show all files

# now save list to a file:
# https://bbs.archlinux.org/viewtopic.php?id=75839
f = open("fileList.TXT", "w")
f.write("\n".join(map(lambda x: str(x), outList))) 
f.close()

 
Regards from:
William Jefferson Photography
514 Daniels St., #211
Raleigh, NC 27605
Cell Phone: (919) 931-6681
EMail: shaggers3@yahoo.com
Brides & Weddings: http://www.WilliamJeffersonPhotography.com
Special Events: http://www.DancingLight.org

---

From: Eric V. Smith <report@bugs.python.org>
To: shaggers3@yahoo.com
Sent: Tuesday, April 10, 2012 5:45 PM
Subject: [bpo-14542] reverse() doesn't reverse sort correctly

Eric V. Smith <eric@trueblade.com> added the comment:

list.reverse() does not reverse a list, it reverses its current values.

Help on built-in function reverse:

reverse(...)
    L.reverse() -- reverse *IN PLACE*

>>

----------
nosy: +eric.smith
resolution:  -> invalid
stage:  -> committed/rejected
status: open -> closed

---

Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue14542\>

---

[mdickinson]

Bill,

list.reverse doesn't do any *sorting* at all; it merely *reverses* the list contents.

>>> x = [1, 3, 4, 2]
>>> x.reverse()
>>> x
[2, 4, 3, 1]

If you want to do a reverse sort, you can either first sort normally and then reverse the result, or (easier) use the 'reverse' keyword argument to the list.sort method, as follows:

>>> x = [1, 3, 4, 2]
>>> x.sort(reverse=True)
>>> x
[4, 3, 2, 1]

I suspect Eric meant to write "does not reverse sort" instead of "does not reverse".

[ericvsmith]

Thanks, Mark.

Indeed, my answer as written is meaningless. I meant it doesn't sort in reverse, it just reverses.

[billje]

Mark and Eric......
Wonderful! I got it now. I used x.sort(reverse=True) and x.sort(reverse=False) and it works just fine. Thanks for your help.
Bill......

 
Regards from:
William Jefferson Photography
514 Daniels St., #211
Raleigh, NC 27605
Cell Phone: (919) 931-6681
EMail: shaggers3@yahoo.com
Brides & Weddings: http://www.WilliamJeffersonPhotography.com
Special Events: http://www.DancingLight.org

---

From: Mark Dickinson <report@bugs.python.org>
To: shaggers3@yahoo.com
Sent: Wednesday, April 11, 2012 8:28 AM
Subject: [bpo-14542] reverse() doesn't reverse sort correctly

Mark Dickinson <dickinsm@gmail.com> added the comment:

Bill,

list.reverse doesn't do any *sorting* at all;  it merely *reverses* the list contents.

[2, 4, 3, 1]

If you want to do a reverse sort, you can either first sort normally and then reverse the result, or (easier) use the 'reverse' keyword argument to the list.sort method, as follows:

[4, 3, 2, 1]

I suspect Eric meant to write "does not reverse sort" instead of "does not reverse".

----------
nosy: +mark.dickinson

---

Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue14542\>

---

[terryjreedy]

Bill, when you reply by email, please snip the signature and quoted message. They are just noise. (Exception: quote a line or two if you are specifically responding to such.) Signatures are inappropriate, and the message you are responding to is already present.