Request for feedback on my first Python program

[Martin Franklin]

Scott Meyers wrote:
> I'm a C++ programmer who's writing his very first Python program.  This
> means the program is going to be gross, and I apologize for that in
> advance.  I don't really have anybody I can show it to for feedback, so I'm
> hoping I can get some comments here.  If there is a better place for me to
> seek guidance, please let me know.
> 
> The program is supposed to read a file containing directory and file names,
> one per line.  (The file can also contain comment and blank lines, which
> should be ignored.)  For each file or directory name, the program should
> print out whether it's a directory, a file, or neither.  That's it.
> 
> Here's the code; you may want to hold your nose:
> 
>   import sys
>   import os
>   import string

as of Python 2.0 the string module is not needed (string's have methods...)



>   
>   # Error status codes
>   BadInvocation = 1

# conventionally  all CAPS for constants
BADINVOCATION = 1


>   
>   def usage(programName):
>     print "Usage: " + os.path.basename(programName) + " [file]"


print "Usage:", os.path.basename(programName), "[file]"
# Normally bad practice to add strings together comma inserts a space so...



>     sys.exit(BadInvocation)
>   
>   # Take a list of strings, return an equivalent list where the following
>   # strings have been removed:
>   # - those with no non-whitespace characters
>   # - those whose first non-whitespace character is a "#"
>   def getMeaningfulLines(lines):
>     nonCommentLines = []
>     for i in range(0,len(lines)):

lines is a python list object so you can iterate over it's items directly

def getMeaningfulLines(lines):
     nonCommentLines = []
     for line in lines:
         if line.startswith("#") or not line:
             continue
         else:
             # call line.strip() to get rid of unwanted \n\r at EOL
             # in fact this is what was causing your program to fail....
             nonCommentLines.append(line.strip())
     return nonCommentLines

>       try:
>         firstToken = string.split(lines[i])[0]
>         if firstToken[0] != "#": 
>           nonCommentLines.append(lines[i])
>       except IndexError:
>         continue
>     return nonCommentLines
>   
>   
>   # if this language had a main(), it'd be here...

# it does..
if __name__=="__main__":
     # this part only executed when called as first arg to python
     # not when imported

>   if len(sys.argv) != 2: usage(sys.argv[0])
>   
>   lines = getMeaningfulLines(open(sys.argv[1]).readlines())
>   for i in lines:
>     print i + " is a ",
>     if os.path.isdir(i): print "directory"
>     elif os.path.isfile(i): print "file"
>     else: print "non-directory and non-file"
> 
> In addition to the numerious stylistic gaffes I'm sure I've made, the
> program also doesn't work correctly.  Given this input file,
> 
>   d:\
>   d:\temp\foo.py
> 
> I get this output:
> 
>   d:\
>    is a  non-directory and non-file
>   d:\temp\foo.py
>    is a  non-directory and non-file
> 
> Aside from being ugly (how do I get rid of the newline that follows each
> directory or file name?), the problem is that the first entry IS a
> directory and the second one IS a file.  So clearly I'm doing something
> wrong.  Any idea what it is?
> 
> Thanks very much in advance.
> 
> Scott



So here it is (tested on my linux machine only....)


import sys
import os


# Error status codes
# conventionally  all CAPS for constants
BADINVOCATION = 1

def usage(programName):
     print "Usage:", os.path.basename(programName), "[file]"
     sys.exit(BADINVOCATION)

# Take a list of strings, return an equivalent list where the following
# strings have been removed:
# - those with no non-whitespace characters
# - those whose first non-whitespace character is a "#"
def getMeaningfulLines(lines):
     nonCommentLines = []
     for line in lines:
         if line.startswith("#") or not line:
             continue
         else:
             nonCommentLines.append(line.strip())
     return nonCommentLines


# if this language had a main(), it'd be here...

# it does.. we just spell it like so:-
if __name__=="__main__":
     # this part only executed when called as first arg to python
     # not when this module is imported

     if len(sys.argv) != 2:
         usage(sys.argv[0])

     lines = getMeaningfulLines(open(sys.argv[1]).readlines())
     for i in lines:
         print i, "is a",
         if os.path.isdir(i):
             print "directory"
         elif os.path.isfile(i):
             print "file"
         else:
             print "non-directory and non-file"



HTH
Martin