= Helper functions for use in Markup templates = [[PageOutline(2)]] Often you need non-trivial presentation logic in templates, but Markup does not (yet) let you drop into straight Python. In Markup, such presentation logic must be either performed in the controller (i.e. the Python code feeding the template with date), or in ''helper functions'' that are called from within template expressions. This page serves as a place where generalized functions that solve common tasks in presentation logic can be collected. At some point, Markup might include a library of such functions. == Python Standard Library == Many of the Python [http://docs.python.org/lib/built-in-funcs.html builtin functions] (such as `reversed` or `sorted`), as well as those in the [http://docs.python.org/lib/module-itertools.html itertools] package (such as `groupby`), can be quite useful in templates. The builtin functions are available by default, whereas other functions need to be put in the template context data explicitly. == group() == The following was written by [http://www.cmlenz.net/ Christopher Lenz] for use in the [http://trac.edgewall.org/ Trac project]: {{{ #!python def group(iterable, num, predicate=None): """Combines the elements produced by the given iterable so that every `n` items are returned as a tuple. >>> items = [1, 2, 3, 4] >>> for item in group(items, 2): ... print item (1, 2) (3, 4) The last tuple is padded with `None` values if its' length is smaller than `num`. >>> items = [1, 2, 3, 4, 5] >>> for item in group(items, 2): ... print item (1, 2) (3, 4) (5, None) The optional `predicate` parameter can be used to flag elements that should not be packed together with other items. Only those elements where the predicate function returns True are grouped with other elements, otherwise they are returned as a tuple of length 1: >>> items = [1, 2, 3, 4] >>> for item in group(items, 2, lambda x: x != 3): ... print item (1, 2) (3,) (4, None) """ buf = [] for item in iterable: flush = predicate and not predicate(item) if buf and flush: buf += [None] * (num - len(buf)) yield tuple(buf) del buf[:] buf.append(item) if flush or len(buf) == num: yield tuple(buf) del buf[:] if buf: buf += [None] * (num - len(buf)) yield tuple(buf) }}} If the `predicate` functionality is not needed, a vastly simpler implementation of that function would be: {{{ #!python def group(iterable, num): """Group an iterable into an n-tuples iterable. Incomplete tuples are discarded e.g. >>> list(group(range(10), 3)) [(0, 1, 2), (3, 4, 5), (6, 7, 8), (9, None, None)] """ return map(None, *[iter(iterable)] * num) }}} See also the Python Cookbook recipe “[http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/303060 Group a list into sequential n-tuples]”. === Example usage === {{{ #!xml
${cell}
}}} That should result in the following output: {{{ #!xml
ab
cd
e
}}} == countoccurrences() == Written by Arnar Birgisson and shared on the IrcChannel. {{{ #!python def countoccurrences(numbers, minlength=0): """Takes a list of integers, in the range of 0..n and returns a list of integers where i-th item is the number of times i appears in the input list. If minlength is specified and n+1 < minlength, the returned list is right-padded with zeroes to make it contain minlength items. Examples: >>> countoccurrences([0,3,1,2,1,1,3,5]) [1,3,1,2,0,1] >>> countoccurrences([0,3,1,2,1,1,3,5], 10) [1,3,1,2,0,1,0,0,0,0] """ # TODO come up with a better name counts = [0] * max(max(numbers)+1, minlength) for x in numbers: counts[x] += 1 return counts }}} === Example usage === {{{ #!xml ${counts[0]} ${counts[1]} ${counts[2]} }}} === Similar version with a dict, counts any hashable types === The previous function can only count occurrences of numbers. This counts occurrences of any hashable types and returns a dict instead of a list. {{{ #!python def countitemoccurrences(items, requireditems=[]): """Takes a list of hashable items and returns a dict whose keys are those items and the values are the counts of how many times each item appers in the list. If the list requireditems is specified it's values are guaranteed to be keys in the resulting dict, even if they don't appear in items in which case the count will be 0 Examples: >>> counttypes('blue green green'.split(), 'blue red green'.split()) {'blue': 1, 'green': 2, 'red': 0} """ counts = dict() for i in requireditems: counts[i] = 0 for i in items: counts[i] = counts.get(i, 0) + 1 return counts }}} ---- See also: MarkupGuide, MarkupRecipes