On Wed, 2009-06-24 at 20:53 +0100, Angus Rodgers wrote: > ... my first Python program! So please be gentle (no fifty ton > weights on the head!), but tell me if it's properly "Pythonic", > or if it's a dead parrot (and if the latter, how to revive it). >
Yay. Welcome to Python. > I'm working from Beazley's /Python: Essential Reference/ (2nd > ed. 2001), so my first newbie question is how best to find out > what's changed from version 2.1 to version 2.5. (I've recently > installed 2.5.4 on my creaky old Win98SE system.) I expect to > be buying the 4th edition when it comes out, which will be soon, > but before then, is there a quick online way to find this out? > Check here: http://docs.python.org/whatsnew/index.html It's not designed to be newbie friendly, but it's in there. > Having only got up to page 84 - where we can actually start to > read stuff from the hard disk - I'm emboldened to try to learn > to do something useful, such as removing all those annoying hard > tab characters from my many old text files (before I cottoned on > to using soft tabs in my text editor). > > This sort of thing seems to work, in the interpreter (for an > ASCII text file, named 'h071.txt', in the current directory): > > stop = 3 # Tab stops every 3 characters > from types import StringType # Is this awkwardness necessary? Not anymore. You can just use str for this. > detab = lambda s : StringType.expandtabs(s, stop) # Or use def First, use def. lambda is a rarity for use when you'd rather not assign your function to a variable. Second, expandtabs is a method on string objects. s is a string object, so you can just use s.expandtabs(stop) Third, I'd recommend passing your tabstops into detab with a default argument, rather than defining it irrevocably in a global variable (which is brittle and ugly) def detab(s, stop=3): #do stuff Then you can do three_space_version = detab(s) eight_space_version = detab(s, 8) > f = open('h071.txt') # Do some stuff to f, perhaps, and then: > f.seek(0) f is not opened for writing, so if you do stuff to the contents of f, you'll have to put the new version in a different variable, so f.seek(0) doesn't help. If you don't do stuff to it, then you're at the beginning of the file anyway, so either way, you shouldn't need to f.seek(0). > print ''.join(map(detab, f.xreadlines())) Sometime in the history of python, files became iterable, which means you can do the following: for line in f: print detab(line) Much prettier than running through join/map shenanigans. This is also the place to modify the output before passing it to detab: for line in f: # do stuff to line print detab(line) Also note that you can iterate over a file several times: f = open('foo.txt') for line in f: print line[0] # prints the first character of every line for line in f: print line[1] #prints the second character of every line > f.close() > > Obviously, to turn this into a generally useful program, I need > to learn to write to a new file, and how to parcel up the Python > code, and write a script to apply the "detab" function to all the > files found by searching a Windows directory, and replace the old > files with the new ones; but, for the guts of the program, is this > a reasonable way to write the code to strip tabs from a text file? > > For writing the output file, this seems to work in the interpreter: > > g = open('temp.txt', 'w') > g.writelines(map(detab, f.xreadlines())) > g.close() > Doesn't help, as map returns a list. You can use itertools.imap, or you can use a for loop, as above. > In practice, does this avoid creating the whole string in memory > at one time, as is done by using ''.join()? (I'll have to read up > on "opaque sequence objects", which have only been mentioned once > or twice in passing - another instance perhaps being an xrange()?) > Not that that matters much in practice (in this simple case), but > it seems elegant to avoid creating the whole output file at once. The terms to look for, rather than opaque sequence objects are "iterators" and "generators". > > OK, I'm just getting my feet wet, and I'll try not to ask too many > silly questions! > > First impressions are: (1) Python seems both elegant and practical; > and (2) Beazley seems a pleasantly unfussy introduction for someone > with at least a little programming experience in other languages. > Glad you're enjoying Beazley. I would look for something more up-to-date. Python's come a long way since 2.1. I'd hate for you to miss out on all the iterators, booleans, codecs, subprocess, yield, unified int/longs, decorators, decimals, sets, context managers and new-style classes that have come since then. > -- > Angus Rodgers Cheers, Cliff -- http://mail.python.org/mailman/listinfo/python-list
