A descriptor for contract attributes

[yaniv256]

Hi Andrea,

I wrote a descriptor for enforcing contracts. From the doc string:

ContractAttribute is a descriptor for object attributes that
enforces a contract check on whatever object or function the
attribute is set to. For a function, the function would not be
passed the self argument (because that breaks encapsulation -
you can still actively pass self if you want). Setting up an
attribute like this is useful when an API that is expressed as
an object requires a client function to work with. In such
cases you want to both communicate expectations with regards
to the needed function and verify that they are met.

Usage example:

from contracts import ContractAttribute, contract, ContractNotRespected

class spam(object): 
    f=ContractAttribute(contract(arg='float,>0')) #for functions
    x=ContractAttribute('float,>0')               #for scalars using string
    y=ContractAttribute(float)                    #for scalars using type

eggs=spam()                                                             

from math import log, exp, pi                                               
eggs.f=lambda (arg): log(arg)                                           

print "eggs.f(e)=" + str(eggs.f(exp(1.0)))                              

try:                                                                    
   print "eggs.f=" + str(eggs.f(-1.0))                                  
except ContractNotRespected as detail:                                  
   print detail                                                         

print "Attempting eggs.x=pi" 
eggs.x=pi
print "eggs.x=" + str(eggs.x) 

print "Attempting eggs.x=-pi" 
try:                                                                    
   eggs.x=-pi                                  
except ContractNotRespected as detail:                                  
   print detail                                                         

print "Attempting eggs.y=2*pi" 
eggs.y=2*pi
print "eggs.y=" + str(eggs.y) 
print "eggs.x=" + str(eggs.x) 

print "Attempting eggs.y=3" 
try:                                                                    
   eggs.y=3                                  
except ContractNotRespected as detail:                                  
   print str(detail)[:180]        

Output:

eggs.f(e)=1.0
Breach for argument 'arg' to <lambda>().
Condition -1.0 > 0 not respected
checking: >0         for value: Instance of float: -1.0   
checking: float,>0   for value: Instance of float: -1.0   
Attempting eggs.x=pi
eggs.x=3.14159265359
Attempting eggs.x=-pi
Condition -3.14159265359 > 0 not respected
checking: >0         for value: Instance of float: -3.141592653589793   
checking: float,>0   for value: Instance of float: -3.141592653589793   
Attempting eggs.y=2*pi
eggs.y=6.28318530718
eggs.x=3.14159265359
Attempting eggs.y=3
Could not satisfy any of the 3 clauses in Float|np_scalar_float|np_scalar,array(float).
 ---- Clause #1:   Float
 | Expected type 'float', got 'int'.
 | checking: Float   for value

[AndreaCensi]

Hi Yaniv,
This looks like something very useful. I'm currently traveling for a few
days so I will check it out in detail this weekend.
A minor comment is that "ContractAttribute" is quite long. What's the
smallest meaningful name for that?

I imagine that this depends on other libraries implementing attributes---I
don't use this design pattern much so I wouldn't know.

a.

On Tuesday, November 18, 2014, Yaniv Ben-Ami [email protected]
wrote:

Hi Andrea,

I wrote a descriptor for enforcing contracts. From the doc string:

A descriptor for object attributes that enforces a contract check on
whatever object or function the attribute is set to. For a function, the
function would not be passed the self argument (because that breaks
encapsulation - you can still actively pass self if you want). Setting up
an attribute like this is useful when an API that is expressed as an object
requires a client function to work with. In such cases you want to both
communicate expectations with regards to the needed function and verify
that they are met.

Usage example:

from contracts import ContractAttribute, contract, ContractNotRespected
class spam(object):
f=ContractAttribute(contract(arg='float,>0')) #for functions
x=ContractAttribute('float,>0') #for scalars using string
y=ContractAttribute(float) #for scalars using type

eggs=spam()
from math import log, exp, pi
eggs.f=lambda (arg): log(arg)
print "eggs.f(e)=" + str(eggs.f(exp(1.0)))
try:
print "eggs.f=" + str(eggs.f(-1.0)) except ContractNotRespected as detail:
print detail
print "Attempting eggs.x=pi"
eggs.x=piprint "eggs.x=" + str(eggs.x)
print "Attempting eggs.x=-pi" try:
eggs.x=-pi except ContractNotRespected as detail:
print detail
print "Attempting eggs.y=2_pi"
eggs.y=2_piprint "eggs.y=" + str(eggs.y) print "eggs.x=" + str(eggs.x)
print "Attempting eggs.y=3" try:
eggs.y=3 except ContractNotRespected as detail:
print str(detail)[:180]

Output:

eggs.f(e)=1.0
Breach for argument 'arg' to ().
Condition -1.0 > 0 not respected
checking: >0 for value: Instance of float: -1.0
checking: float,>0 for value: Instance of float: -1.0
Attempting eggs.x=pi
eggs.x=3.14159265359
Attempting eggs.x=-pi
Condition -3.14159265359 > 0 not respected
checking: >0 for value: Instance of float: -3.141592653589793
checking: float,>0 for value: Instance of float: -3.141592653589793
Attempting eggs.y=2*pi
eggs.y=6.28318530718
eggs.x=3.14159265359
Attempting eggs.y=3
Could not satisfy any of the 3 clauses in Float|np_scalar_float|np_scalar,array(float).
---- Clause #1: Float
| Expected type 'float', got 'int'.
| checking: Float for value

---

You can merge this Pull Request by running

git pull https://github.com/yaniv256/contracts ContractAttribute

Or view, comment on, or merge it at:

#30
Commit Summary

• before revert on backport.py
• Added support for callable objects and bound methods
• ContractAtribute added to main
• Added doc string
• Changed the name of the private attribute used to store wrapped
  values to contracts
• a bit more info in doc string
• Added support for scalar attributes

File Changes

• M src/contracts/init.py
  https://github.com/AndreaCensi/contracts/pull/30/files#diff-0 (2)
• M src/contracts/main.py
  https://github.com/AndreaCensi/contracts/pull/30/files#diff-1 (109)

Patch Links:

• https://github.com/AndreaCensi/contracts/pull/30.patch
• https://github.com/AndreaCensi/contracts/pull/30.diff

—
Reply to this email directly or view it on GitHub
#30.

(sent from mobile device, please pardon typos and brevity)

[yaniv256]

Thanks!

I guess we could go with something like "verify", "guard", "watch" or "attach". It would be nice to have it sound like an original python keyword.

No, I used just functools and types, both from the standard library.

[AndreaCensi]

On Tue, Nov 18, 2014 at 8:07 PM, Yaniv Ben-Ami [email protected]
wrote:

No, I used just functools and types, both from the standard library.

Sorry let me reword my comment: I meant, if there is a popular library
implementing descriptors, then the syntax in PyContracts should be similar
to it. I will look at the topic when I come back home.

thanks for the contribution!

A.

Andrea Censi | LIDS / MIT | http://censi.mit.edu

[yaniv256]

BTW, I learned how to do descriptors from Chris Beaumont's excellent writeup (which I found via google).

Chris - I changed your design pattern a bit to hold the data in a dict on the instances, rather than on the descriptors. I hope it is as valid, but it would be good if you review.

[AndreaCensi]

@ChrisBeaumont you are the expert with descriptors. What do you think of this implementation?

[ChrisBeaumont]

This looks good to me! This is very similar to how Traits works (http://code.enthought.com/projects/traits/)

I have a few style comments that I'll leave inline.

[ChrisBeaumont]

Any reason to not use function_.__name__ = class_name +'.__call__' here?

[yaniv256]

For some reason it generates an error:

AttributeError: 'instancemethod' object has no attribute 'name'

[ChrisBeaumont]

@AndreaCensi regarding Traits and what to name these:

IPython has a "traitlets" module they use, whose API is similar to traits: http://ipython.org/ipython-doc/dev/api/generated/IPython.utils.traitlets.html

The problem is that both of these libraries are more "MyPy"-style, in that they have many special-purpose classes like Float, Boolean to express type info, rather than the DSL-style approach contracts uses. So they don't provide a precedent for whether to call these things Properties or something else.

I personally like Property or Attribute

[yaniv256]

OK. For my own project I need functions, so I'll use my own fork until the arrow notation is ready. But for the pull request I scaled back function support. It's a trivial descriptor now, and should be fairly robust.

I also renamed to Attribute.

[AndreaCensi]

I tried to merge this, but the two changes you made to main.py (one involving using partial functions) make tests fail in nonobvious ways.

I'm working on this branch: https://github.com/AndreaCensi/contracts/tree/yaniv256-ContractAttribute
You can set Yaniv = False or True alternatively to enable/disable your changes.

Run nosetests contracts to see what tests fail.