Tuesday, November 10, 2015

Formatting Strings and the str.format() family of functions -- Python 3.4 Notes

I have to be clear that I am obsessed with the str.format() family of functions. I've happily left the string % operator behind. I recently re-discovered the vars() function.

My current go-to technique for providing debugging information is this:

print( "note: local={local!r}, this={this!r}, that={that!r}".format_map(vars)) )

I find this to be handy and expressive. It can be replaced with logging.debug() without a second thought. I can readily expand what's being dumped because all locals are provided by vars().

I also like this as a quick and dirty starting point for a class:

def __repr__(self):
    return "{__class__.__name__}(**{state!r})".format(__class__=self.__class__, state=vars(self))

This captures the name and state. But. There are nicer things we can do. One of the easiest is to use a helper function to reformat the current state in keyword parameter syntax, like this:

def args(obj):
    return ", ".join( "{k}={v!r}".format(k=k,v=v) for k,v in vars(obj).items())

This allows us to dump an object's state in a slightly nicer format. We can replace vars(self) with args(self) in our __repr__ method. We've dumped the state of an object with very little class-specific code. We can focus on the problem domain without having to wrestle with Python considerations.

Format Specifications

The use of !r for formatting is important. I've (frequently) messed up and used things like :s where data might be None. I've discovered that -- starting in Python 3.4 -- the :s format is unhappy with None objects. Here's the exhaustive enumeration of cases. 

>>> "{0} {1}".format("s",None)
's None'
>>> "{0:s} {1:s}".format("s",None)
Traceback (most recent call last):
  File "", line 1, in 
    "{0:s} {1:s}".format("s",None)
TypeError: non-empty format string passed to object.__format__
>>> "{0!s} {1!s}".format("s",None)
's None'
>>> "{0!r} {1!r}".format("s",None)
"'s' None"

Many things are implicitly converted to strings. This happens in a lot of places. Python is riddled with str() function evaluations. But they aren't everywhere. Python 3.3 had one that was removed for Python 3.4 and up.

Bottom Line: be careful where you use :s formatting.  It may do less than you think it should do.

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