pycheck 0.1

Type checking tools for debugging and developing Python code

checked(f)
checked is a function decorator that uses function annotations as type declarations
and verifies that the function has been passed, and is returning, the
correct types of values.

Yes, yes, I know, this is python and we're not supposed to have to have
type checking. Before you pound your keyboard angrily telling me what an
idiot I am, please at least read the "Motivation" section below. Thank you.

SAMPLE USAGE:
------------

@checked
def function(parameter:<type declaration>) -> <type declaration>
....

"function" may be a function, a method, or a generator*.

* For generators the actual return type is type <generator> however
@checked will check that the values yielded by the generator are
of the specified type.

<type declaration> may be any of the following:

* a class or type object (int, str, MyClass, etc.)

Example:
``def f( x:int ): # f() only accepts int``

* A tuple (or any iterable) of type objects, indicating that the value may be an instance of
any of those types.

Example:
``def f( x: (int, str) ): # f() must return a str OR None``

* The literal None, to indicate that only None is an acceptable value
(usually used for declaring that a function does not return a value)

Example:
``def f( x ) -> None: # f() has no return value``

* A tuple (or any other iterable) containing type objects and the value None, indicating that
the value may be any of the indicated types or the special value None.

Example:
``def f( x ) -> (str, None) # f() must return a str or None``

* Dict (or other mapping type) object which maps a collection type to an element type,
{<collection type> : <element type>}.

This usage means that the function ruturns a collection of type <collection type>
(e.g. set, list, tuple, etc.), which contains only elements of type <element type>.

Example:
``def f(x) -> {set:int}: # f() must return a set of integers``

* A callable object which accepts one parameter and returns true or false. The callable
object is treated as a pre-condition when annotating function parameters and a
post-condition when annotating the return value. The value passed or returned will be
passed to the callable and an error will be raised if the result is not true.

Example
``def f( x: lambda x: x > 0 ): # f() only accepting values greater than zero``

``def g(x) -> lambda y: y >= 0: # g() must return a value >= 0``


DEFAULTS, *ARG, AND **KWD
--------------------------------------
Parameters with default values, as well as *arg and **kwd arguments can also have
their types declared:

@checked
def function(x:<type declaration>, y:<type declaration>=1, *args:<type declaration>, **kwds:<type declaration):


DEBUG MODE:
-----------------------------
@checked is intended to be used as a tool during
development and testing to help identify errors more quickly. For
this reason ``@checked`` is only functional in debug mode
(i.e. when ``__debug__ == True``). If not in debug mode it is defined
simply as,

def checked(f):
return f


MOTIVATION:
-----------
The original motivation for this function was my attempt to apply the excercises outlined in Ben Nadel's whitepaper,
"Object Calisthenics" (http://www.bennadel.com/resources/uploads/2012/ObjectCalisthenics.pdf).

One of Nadel's excercises is to, "wrap all primitives and Strings." What he means is that instead of passing simple
ints, floats, and strings to a function, you'd create simple subclasses of int, float and str to represent the
type of data the parameter actually represents and to protect you from accidently passing an integer that
represents Hours to a function that is intended to work with dollar ammounts, for example.

Of course, Nadel wrote his paper with Java in mind, so the java compiler would actually catch an error like that.
In attempting to apply that recommendation to python something was needed in order implement type checking.
Not finding a suitable package after an exhaustive five minute search of the internet I decided to try to
implement my one on my own (okay, I really wanted to see if I could do it or not).


LIMITATIONS
-----------
* there's currently no way to verify if the default value specified for a parameter violates the
type declaration for that parameter.
Example:
def f( x:int="bad default value" ):
....

Hopefully the fact that the type declaration is RIGHT THERE IN FRONT OF YOU will be enough to
get you to use the right type.

This actually has at least one legitamate use, actually, in the case where the default value
is a magic value (typically None) which indicates that the user didn't supply a value:

def f( x:int=None )
# user is never intended to specify a value of None. User is expected
# to either supply an int or no value at all.

if x is None:
...do something special...

* If the annotations are callable objects, they can only operate on a single parameter, or the return,
value. The validation cannot evaluate all of the parameters -- for example you can't write a
pre-condition that checks that the parameters are ordered from least to greatest.


KNOWN ISSUES:
-------------
Although an error is raised as it should be, the text of the error message for a container containing and
element of the wrong type is not clear

type_proxy(typename)
type_proxy(typename) is used when an actual typename cannot be used because
it is not yet defined in the namespace and cannot be.

The two typical causes are,

1. Using a class in an annotation of a method of that same class.
Example:
class A:
@checked
@classmethod
def factory(cls, ...) -> A: # Error, when the annotation is processed, A has not yet been defined
...
Solution:
class A:
@checked
@classmethod
def factory(cls, ...) -> type_proxy('A') # valid
...

2. Trying to create an annotation which refers to a class not yet defined, which itself uses an annotation
that refers back to _this_ class (i.e. circular references). Because the two classes refer to each other
the solution is not so simple as to just move the second class definition before the first.

Example:
class A:
@checked
def to_b(self) -> B : # Error: B not yet defined
return B(self)

class B:
@checked
def to_a(self) -> A : # A refers to B and B refers to A, so it doesn't matter
return A(self) # which class is defined first, there's going to be an error.

Solution:
class A:
@checked
def to_b(self) -> type_proxy("B") : # Fixed
return B(self)

class B:
...unchanged...