Pretty cool, I worry that such things can’t easily be done so cleanly in python…

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I recently wrote a plugin for nosetests which greatly (imho) improves the output for failed and errored tests. The screenshot explains it best.

To install, just run easy_install rednose. Then you can run nosetests with the –rednose option.

See github.com/gfxmonk/rednose for code and more information.

More and more, I am wishing that there was some sort of strict mode I could enable in ruby to say “you know what? I’m careful with my code. Please don’t assume things behind my back”. And to be honest, this mode would pretty much be synonymous with “just do what python would do”

By default, python is strict. If you index a dict (hash) with a nonexistant key, you get a keyError. If you don’t want to have to deal with that, you can use the get method and provide a default for if the key doesn’t exist. In ruby, if you want to be strict about anything, you generally have to write your own checks to guard against the core library’s forgivingness. Forgivingness sounds nice at first, but goes completely against the idea of failing fast, and frequently delays the manifestation of bugs, making them that much harder to actually track down.

Two examples that I came across within minutes of each other the other day:

Struct.new(:a,:b,:c).new('a','b')

that should NOT go without an exception

"a|b||c".split("|")
=> ["a", "b", "", "c"]

good…. so now:

"a|b||".split("|")
=> ["a", "b"]

argh! what have you done to my third field?

On the same codebase, with no changes pending in either system:

$ time git status
# ...
real	0m0.618s

$ time bzr status
# ...
real	0m3.795s

It’s a small thing, but it matters.

Python decorators are cool, but they can become very messy if you sart taking arguments:

def decorate_with_args(arg):
    def the_decorator(func):
        def run_it():
            func(arg)
        return run_it
    return the_decorator

@decorate_with_args('some string')
def messy(s):
    print s

ugh. Three levels of function definitions for a single decorator. And heaven forbid you want the decorator to be useable without supplying any arguments (not even empty brackets).

So then, I present a much cleaner decorator helper class:

class ParamDecorator(object):
    def __init__(self, decorator_function):
        self.func = decorator_function

    def __call__(self, *args, **kwargs):
        if len(args) == 1 and len(kwargs) == 0 and callable(args[0]):
            # we're being called without paramaters
            # (just the decorated function)
            return self.func(args[0])
        return self.decorate_with(args, kwargs)

    def decorate_with(self, args, kwargs):
        def decorator(callable_):
            return self.func(callable_, *args, **kwargs)
        return decorator

All that’s required of you is to take the decorated function as your first argument, and then any additional (optional) arguments. For example, here’s how you might implement a “pending” and “trace” decorator:

@ParamDecorator
def pending(decorated, reason='no reason given'):
    def run(*args, **kwargs):
        print "function '%s' is pending (%s)" % (decorated.__name__, reason)
    return run

@ParamDecorator
def trace(decorated, label=None):
    if label is None:
        label = decorated.__name__
    else:
        label = "%s (%s)" % (decorated.__name__, label)
    def run(*args, **kwargs):
        print "%s: started (args=%s, kwargs=%s)" % (label, args, kwargs)
        ret = decorated(*args, **kwargs)
        print "%s: returning: %s" % (label, ret)
        return ret
    return run

Which can then be used as either standard or paramaterised decorators:

@pending
def a():
    pass

@pending("I haven't done it yet!")
def b():
    pass

@trace
def foo():
    return "blah"

@trace("important function")
def bar():
    return "blech!"

And just to show what this all amounts to:

if __name__ == '__main__':
    a()
    b()
    foo()
    bar()

reveals the following output:

function 'a' is pending (no reason given)
function 'b' is pending (I haven't done it yet!)
foo: started (args=(), kwargs={})
foo: returning: blah
bar (important function): started (args=(), kwargs={})
bar (important function): returning: blech!

Fairly simple stuff, but hopefully useful for anyone who finds themselves tripped up by decorators - particularly when trying to allow for both naked and paramaterised decorators.

P.S: I’ve made a pastie of the code in this post, because my weblog engine is not cool enough to colour-code python ;)

A snippet from the source code of my iPhone app, GRiS:

- (void) tableView: (UITableView*) tableView
willBeginEditingRowAtIndexPath: (NSIndexPath *) indexPath {
	// the mere presence of this method causes a swipe action to be
	// recognised, and a delete button appears. like magic!
}

I’m glad it was so easy, but fairly unnerved at the same time…