Dictionary isn't emptying...

[mikeserv]

I've been reading a few Python books for the past week or so and I'm trying to gain some simple programming skills. I've written a program that will recursively scan through a folder and remember all of the files inside of it.

I think I will eventually use the script to sync my Rockbox iPod, but for now it's just an excercise.

Here's the code:

#!/usr/bin/python
#Filename : Beginnings.py

modules = {}
import sys
import os
import time
import cPickle as p

def mp3_dict(path, mp3_list = {}):
"""Scans a path for all .mp3 files. Returns dictionary 'path:file;file;file"""
folder = [] #defines folder as list
for item in os.listdir(path): #scans path for all files and folders
if item[0] == ".": #ignores hidden files
pass
elif os.path.isfile(path + item) and item[-3:] == "mp3": #checks if item is mp3 file
try: #checks if mp3 is first in folder and adds to value list
mp3_list[path] = mp3_list[path] + ";" + item
except:
mp3_list[path] = item
elif os.path.isdir(path + item + "/"): #checks if item is folder, and adds it to sub
folder.append(path + item + "/") #folder list
[mp3_dict(item, mp3_list) for item in folder] #recursively scans subfolder list
return mp3_list

def dat_write(dat_path, f_name):
"""Pickles return of mp3_dict for dat_path to file f_name"""
f = open(dat_path + f_name, 'w')
p.dump(mp3_dict(dat_path), f)
f.close()

def dat_read(path):
"""Unpickles and returns path."""
f = file(path)
dat = p.load(f)
f.close()
return dat

def dat_print(mp3l, path):
"""Parses out dictionary in format path:file;file;file
and prints readable output."""
print path + ":\n" #prints original search path
for row in mp3l:
print row[len(path):] + ": ", #prints subfolder name
for item in mp3l[row].split(";"): #splits dictionary values into filenames
if item == mp3l[row].split(";")[-1]: #checks if file is last in folder
print "%s\n" %(item) #prints last file and starts new line
else:
print "%s, " %(item), #prints files in subfolder in comma sep list

ipath = '/media/ipod/PodCasts/'
hpath = '/home/mhodgin/PodCasts/'
sfile = 'mp3_list.dat'

dat_print(mp3_dict(ipath), ipath)
dat_print(mp3_dict(hpath), hpath)

#dat_write(hpath, sfile)
#dat_write(ipath, sfile)
#dat_print(dat_read(hpath + sfile), hpath)
#dat_print(dat_read(ipath + sfile), ipath)


The problem I'm having is this: the first (and main) function mp3_dict() scans some folders and saves its files to the dictionary fine, but for some reason it doesn't clear the dictionary afterwards. So when I run it the second time, it returns a compounded list, and this is frustrating. Why isn't the variable clearing?

-Mike

[pmasiar]

Without even trying, you invented advanced trick by declaring mp3_list as static variable. Why you need to pass it in? Instead, don't try to be clever, declare it as local variable inside function (and not as default parameter). And read about how you can update dictionary with another dictionary.

Consider also more flexible data structures, like dict which values are lists (instead of string with ';' as separator).

Nitpicking about naming:
- variable mp3_list is a dictionary. WTF?
- functions are more natural with verb-based name: make_mp3s (returns list or dict of something)

[mikeserv]

Ok, I've taken your advice about the naming and making the dictionary values a list, but that doesn't fix my original problem. How did I declare the variable static? Why doesn't it clear when the function is called without the mp3_dict argument? Without that argument, shouldn't the variable be equal to {}?


#!/usr/bin/python
#Filename : Beginnings.py

modules = {}
import sys
import os
import time
import cPickle as p

def make_mp3_dict(path, mp3_dict = {}):
"""Scans a path for all .mp3 files. Returns dictionary 'path:file;file;file"""
folder = [] #defines folder as list
flist = []
for item in os.listdir(path): #scans path for all files and folders
if item[0] == ".": #ignores hidden files
pass
elif os.path.isfile(path + item) and item[-3:] == "mp3": #checks if item is mp3 file
flist.append(item)
elif os.path.isdir(path + item + "/"): #checks if item is folder, and adds it to sub
folder.append(path + item + "/") #folder list
if flist != []:
mp3_dict[path] = flist
[make_mp3_dict(item, mp3_dict) for item in folder] #recursively scans subfolder list
return mp3_dict

def dat_write(dat_path, f_name):
"""Pickles return of mp3_dict for dat_path to file f_name"""
f = open(dat_path + f_name, 'w')
p.dump(make_mp3_dict(dat_path), f)
f.close()

def dat_read(path):
"""Unpickles and returns path."""
f = file(path)
dat = p.load(f)
f.close()
return dat

def dat_print(mp3l, path):
"""Parses out dictionary in format path:file;file;file
and prints readable output."""
print path + ":\n" #prints original search path
for row in mp3l:
print row[len(path):] + ": ", #prints subfolder name
for item in mp3l[row]: #splits dictionary values into filenames
if item == mp3l[row][-1]: #checks if file is last in folder
print "%s\n" %(item) #prints last file and starts new line
else:
print "%s, " %(item), #prints files in subfolder in comma sep list

ipath = '/media/ipod/PodCasts/'
hpath = '/home/mhodgin/PodCasts/'
sfile = 'mp3_dict.dat'

dat_print(make_mp3_dict(ipath), ipath)
dat_print(make_mp3_dict(hpath), hpath)

#dat_write(hpath, sfile)
#dat_write(ipath, sfile)
#dat_print(dat_read(hpath + sfile), hpath)
#dat_print(dat_read(ipath + sfile), ipath)


-Mike

[mikeserv]

A guy at python-forum.org took a look at the code and gave me this link: http://www.python.org/doc/faq/general/#why-are-default-values-shared-between-objects

My code now looks like this (and it works correctly):


#!/usr/bin/python
#Filename : Beginnings.py

modules = {}
import sys
import os
import time
import cPickle as p

def make_mp3_dict(path, mp3_dict = None):
"""Scans a path for all .mp3 files. Returns dictionary 'path:filelist"""
folder = [] #defines folder as list
flist = []
if mp3_dict == None:
mp3_dict = {}
for item in os.listdir(path): #scans path for all files and folders
if item[0] == ".": #ignores hidden files
pass
elif os.path.isfile(path + item) and item[-3:] == "mp3": #checks if item is mp3 file
flist.append(item)#adds file to filelist
elif os.path.isdir(path + item + "/"): #checks if item is folder, and adds it to sub
folder.append(path + item + "/") #folder list
if flist != []:#only adds folders containing .mp3's to mp3_dict
mp3_dict[path] = flist
[make_mp3_dict(item, mp3_dict) for item in folder] #recursively scans subfolder list
return mp3_dict


def dat_write(dat_path, f_name):
"""Pickles return of mp3_dict for dat_path to file f_name"""
f = open(dat_path + f_name, 'w')
p.dump(make_mp3_dict(dat_path), f)
f.close()

def dat_read(path):
"""Unpickles and returns path."""
f = file(path)
dat = p.load(f)
f.close()
return dat

def dat_print(mp3l, path):
"""Parses out dictionary in format path:filelist
and prints readable output."""
print path + ":\n" #prints original search path
for row in mp3l:
print row[len(path):] + ": ", #prints subfolder name
for item in mp3l[row]: #splits dictionary value lists
if item == mp3l[row][-1]: #checks if file is last in folder
print "%s\n" %(item) #prints last file and starts new line
else:
print "%s, " %(item), #prints files in subfolder in comma sep list

ipath = '/media/ipod/PodCasts/'
hpath = '/home/mhodgin/PodCasts/'
sfile = 'mp3_dict.dat'

dat_write(hpath, sfile)
dat_write(ipath, sfile)
dat_print(dat_read(hpath + sfile), hpath)
dat_print(dat_read(ipath + sfile), ipath)


-Mike