This is a function to iterate over a container and its elements that checks for recursive traps. The condition for descending into elements is highly configurable (a list of type() results, or a callable). Spin-offs: a function to check for iterability in a loose and in a strict sense; a function to flatten a hierarchical container into a list of its elements.
Python, 143 lines
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def iterable( x, strict = 0 ): """Return true if +x+ allows application of the +in+ operator (which excludes, for example, numbers and +None+), *and* the +in+ loop is actually entered into (excluding sequences of length zero). If +strict+ is true, an affirmative answer is furthermore subject to the condition that the first element retrieved within an +in+ loop be not equal to the thing being iterated over, which excludes strings of length one. -- Used by +walk()+ and +flatten()+.""" try: for element in x: if strict: return element != x return 1 except TypeError: pass return 0 class RecursiveContent: """Wrapper class for content classed as displaying recursive behavior. The original content is available as ~+.data+. Given content +foo+ that prints a symbolic representation like, say, +^blah$+, the representation +`RecursiveContent(foo)+ is +^...$+, ie, the surround characters are kept, while the inner part is replaced by a typographical ellipsis. This is somewhat compatible with the way Python symbolizes a recursive list. -- Used by +walk()+ and +flatten()+.""" def __init__( self, data ): self.data = data def __call__( self ): return self.data def __repr__( self ): return '%s...%s' % ( `self.data`, `self.data`[-1] ) def __iter__( self ): return iter( self.data ) def walk( seq, descend = ( list, tuple ), ancestry = None ): """This function provides a convenient and recursion-proof way to iterate over arbitrary data (which is a surprisingly involved task). +seq+ may be anything accepting the application of the +in+ operator; +descend+ should either be a sequence of types that will be descended into, or a function that decides, when called with a sequence element as argument, whether to descend into that element or not. +ancestry+ is an argument used by +walk()+ for internal bookkeeping (but you can manipulate it for special effects). -- Note that +descend+ is only checked when trying to decide whether to descend into *elements* of the sequence given, not when iterating over the sequence itself. Hence, if you pass a tuple but specify that only lists should be descended into, the outermost sequence will still be analyzed into elements. For example:: seq = ( 0, 1, 2, [ 3, 4 ], 5, ( 6, 7 ), 8 ) for e in walk( seq, ( list, ) ): print e, will print :: 0 1 2 3 4 5 (6, 7) 8 onto the screen. It is possible to pass +str+ as an element in +descend+, which will split strings into characters, so the results of :: walk( ( 0, 'abc', 1 ), ( str, ) ) and walk( ( 0, 'a', 'b', 'c', 1 ) ) become indistinguishable. Note that data whose type appears in +descend+ but which are inherently non-iterable will simply be absent from the result. This is logical since iterables whose types are listed in +descend+ but happen to be empty likewise do not leave any trace in the result (since they don't offer elements to be looped over). As convenient spin-offs, there are two more functions related to walk: +iterable()+ and +flatten()+, q.v. IMPLEMENTATION ============== The fundamental function is as simple as this:: def walk( seq, descend ): for element in seq: if descend( element ): for subelement in walk( element, descend ): yield subelement else: yield element However, this definition is not safe if an iterable contains itself as an element. In order to prevent the function from descending infinitely, we need to keep an account of what elements we have already come across in the same branch; argument +ancestry+ does exactly that (it is o.k. to put copies of he same container side by side into another container, since that does not lead to recursion; we only need check for cases where a container contains itself). If a recursive element is detected, an instance of +RecursiveContent+ is returned (that the caller might want to check for). +RecursiveContent+s print out much the same way Python's standard representation of recursive lists does. Another difficulty comes with strings: when a string is being iterated over, it yields characters. Now, a character is simply a string of length one, so once we allow descending into strings on the ground of their type, we also allow to descend into characters, which would result in recursion. To prevent this, +walk()+ additionally checks for 'strict iterability', as defined by function +iterable()+ when called with +strict=1+. THANKSTO ======== This code was inspired by the posting "Walk a directory tree using a generator" by Tom Good, http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/105873. """ # If descend is not already a function, construct one that # assumes that descend has been given as a list of types: _descend = descend if not callable( _descend ): _descend = lambda x: type( x ) in descend # Initialize ancestry: if ancestry is None: ancestry =  # Register sequence as ancestor: ancestry.append( seq ) # Unconditionally loop over all elements in sequence: for element in seq: # If element is in ancestry: if element in ancestry: yield RecursiveContent( element ) # If element is not in ancestry: else: # If element is eligible for descend, see whether to actually descend # into it, yield it wholly, or don't yield anything: if _descend( element ): # If element is eligible and strictly iterable (i.e., not inherently # recursive and not an empty sequence), descend: if iterable( element, strict = 1 ): for subelement in walk( element, descend, ancestry ): yield subelement # If element is eligible and not strictly, but at least loosely iterable, # yield it (because then it is something like a single-character string # which should not be lost): elif iterable( element ): yield element # If element is not eligible for descend, yield it: else: yield element ancestry.pop() def flatten( seq, descend = ( list, tuple ) ): """Return a list of all the elements that result from an exhaustive +walk()+ over a given +seq+uence. See +walk()+ for the interpretation of +descend+.""" return [ element for element in walk( seq, descend ) ]
Backquotes in __repr__. In the class RecursiveContent, there are backquotes around the data references:
What is the purpose of the backquotes?