On Jul 15, 7:58 pm, Karsten Wutzke <kwut...@web.de> wrote: > Hello, > > this is obviously a Python OO question: > > Since Python isn't stringly typed, single-dispatch isn't available per > se. So is the "double-dispatch" Visitor pattern, which is usually used > in OO systems to implement code generators. So, what is the de facto > method in Python to handle source code generation? > > Karsten
You ask about code generation and you already had answers in that area, but let me talk a bit about a simpler topic, traversing a hierarchical file system. I think this is relevant (even if not answering your question) if you want to get familiar with the Python way. In the old days, the way to traverse a file system was through the os.path.walk function. Here is what the docs say (from http://docs.python.org/library/os.path.html): """ os.path.walk(path, visit, arg) Calls the function visit with arguments (arg, dirname, names) for each directory in the directory tree rooted at path (including path itself, if it is a directory). The argument dirname specifies the visited directory, the argument names lists the files in the directory (gotten from os.listdir(dirname)). The visit function may modify names to influence the set of directories visited below dirname, e.g. to avoid visiting certain parts of the tree. (The object referred to by names must be modified in place, using del or slice assignment.) """ As you see the documentation make explicit reference to the visitor pattern. However a note below says: """ This function is deprecated and has been removed in 3.0 in favor of os.walk(). """ In other word, the visitor pattern is *not* the Pythonic way to solve this problem. The Pythonic way is to use os.walk, which converts the nested structure in a flat structure. From the docs (http://docs.python.org/library/os.html): """ This example displays the number of bytes taken by non-directory files in each directory under the starting directory, except that it doesn’t look under any CVS subdirectory: import os from os.path import join, getsize for root, dirs, files in os.walk('python/Lib/email'): print root, "consumes", print sum(getsize(join(root, name)) for name in files), print "bytes in", len(files), "non-directory files" if 'CVS' in dirs: dirs.remove('CVS') # don't visit CVS directories """ There is a big conceptual difference between os.path.walk and os.walk. The first works like a framework: you pass a function to it and os.path.walk is in charging of calling it when needed. The second works like a library: os.walk flattens the hierarchical structure and then you are in charge of doing everything you wish with it. os.walk is the Pythonic way, and you suggested to follow that approach; for instance elementTree and lxml (libraries for parsing XML data) work exactly that way. Actually one of the motivating examples for the introduction of generators in Python was their use in flattening data structure, i.e. exactly the pattern used by os.walk. The message is stop thinking like in Java and start using idiomatic Python. We are here to help. Michele Simionato -- http://mail.python.org/mailman/listinfo/python-list
