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strict 0.1alpha

Type and subject matter checks for functions and classes.

strict is a module, that addresses the frequent need for type checking in Python.

Quick Example

Function Constraints

Add type constraints to the function signature:

from __future__ import print_function
from strict.functions import *

@expects(arg(float), arg(float), message=arg(str))
def multiply_and_print(a, b, message="You've got {result}"):
    print(message.format(result=(a * b)))

# This works just fine
multiply_and_print(2.0, 3.0)

# This raises a TypeConstraintError
multiply_and_print(int(2.0), 3.0)

Add type constraint to a method:

from __future__ import print_function
from strict.functions import *

class Foo(object):

    @expects(self, arg(str))
    def __init__(self, name):
        self.name = name

    def say_hello(self):
        print(", ".join(["Hello", self.name]))

# This works
f = Foo("Jim")
f.say_hello()

# This doesn't
f = Foo(123)
f.say_hello()

Add a constraint on the return value of a function:

from strict.functions import *

@returns(int)
def return_two():
    return 2

@returns(int)
def return_half():
    return 0.5

# This works fine
return_two()

# This raises an exception
return_half()

Add a subject matter constraint, as well as a type constraint to a function:

@expects(arg(float), arg(float, lambda x: x != 0))
def divide(a, b):
    "We need to make sure, that b isn't zero..."
    return a / b

# This works
divide(4.0, 2.0)

# This all fails with a TypeConstraintError
divide(4, 2.0)
divide(4.0, "poop")

# This fails with a SubjectConstraintError
divide(4.0, 0.0)

Structures

Define a structure:

from strict.structures import *

class Point(Structure):
    """
    Let's have a point, that can fit only on a certain area.
    """
    x = Field(float, lambda x: 0.0 < x < 1000.0)
    y = Field(float, lambda y: 0.0 < y < 1000.0)

    def __init__(self, x, y):
        self.x = x
        self.y = y

# This works
a = Point(10.0, 15.0)

# This all fails with the correct exception
Point(10, 15)
Point(10.0, 1001.0)
Point(-2.0, 50.0)

Serialize your structure:

from strict.structures import *

class Point(Structure):
    """
    Let's have a point, that can fit only on a certain area.
    """
    x = Field(float, lambda x: 0.0 < x < 1000.0)
    y = Field(float, lambda y: 0.0 < y < 1000.0)

    def __init__(self, x, y):
        self.x = x
        self.y = y

a = Point(10.0, 15.0)

# This outputs a dictionary
a.to_dict()

# This outputs a list
a.to_list()

Why Would I Type Check?

Although, the common idiom to deal with type compatibility issues in Python is duck typing, in some cases it is not adequate.

Quite often the information about the usage context is implicitly encoded in the class name, which duck typing would usually fail to handle, unless you introduce wildly differing class interfaces or just kludge.

Consider the following class tree of a financial application:

                    +--------------------------+
                    |                          |
                    |   BaseVolatilitySurface  |
                    |                          |
                    +-------------+------------+
                                  |
                                  |
                                  |
                                  |
+---------------------------+     |     +-------------------------+
|                           |     |     |                         |
|  EquityVolatilitySurface  <-----+----->  RateVolatilitySurface  |
|                           |           |                         |
+---------------------------+           +-------------------------+

Volatility of some value (usually of an asset price) is the key input of valuation models for financial derivatives. In this case, we try to model in our application the volatility of equity and interest rates, implied by market prices.

Both classes would have mostly the same interface. If we rely on duck typing and accidentaly pass a RateVolatilitySurface to value an equity option, no exception would be raised, since techically the input would satisfy the constraint expressed through duck typing. However, the result would be wrong and we’d get a typical example of a SH*T IN, SH*T OUT program.

The obvious solution is to sprinkle your code with a liberal amount of isinstance checks. This, however, quickly leads to your business code being dominated by boilerplate checks and error handling. To avoid this I tried to put the most useful and simple patterns in a module:

  • Assigning constraints to function arguments and return values
  • Creating a class with strict type checking of its properties and the correctly filled __slots__ variable.

2014, Jevgeni Tarasov (jevgeni@tarasov.ch)

 
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