00_example.py

from contracts import contract, invariant, precondition, postcondition

import pytest

@precondition(lambda ctx: ctx[0] == 1)       # positional arguments
@precondition(lambda ctx: ctx['two'] == 2)   # named arguments
@precondition(lambda ctx: ctx['three'] == 3, 'Onoes !') # with a message
@postcondition(lambda ctx: ctx['return'] == 1 + 2 + 3) # return
def plop(one, two, three):
  return one + two + three


plop(1, two=2, three=3)
plop(1, two=3, three=3)
# ^^^^^^^^^^^
# Traceback (most recent call last):
#  File "test.py", line 15, in <module>
#    plop(1, two=3, three=3)
#  File "/Users/thomas/projects/tradebot/app/dummybot/contracts.py", line 46, in inner
#    res = func(*args, **kwargs)
#  File "/Users/thomas/projects/tradebot/app/dummybot/contracts.py", line 44, in inner
#    assert pre(ctx), "Pre-condition failed: {}".format(get_message(pre, pre_msg))
# AssertionError: Pre-condition failed: @precondition(lambda ctx: ctx['two'] == 2)   # named arguments

plop(1, two=2, three=4)
# ^^^^^^^^^^^^^^^^^^^^^
# Traceback (most recent call last):
#  File "test.py", line 15, in <module>
#    plop(1, two=2, three=4)
#  File "/Users/thomas/projects/tradebot/app/dummybot/contracts.py", line 46, in inner
#    res = func(*args, **kwargs)
#  File "/Users/thomas/projects/tradebot/app/dummybot/contracts.py", line 46, in inner
#    res = func(*args, **kwargs)
#  File "/Users/thomas/projects/tradebot/app/dummybot/contracts.py", line 44, in inner
#    assert pre(ctx), "Pre-condition failed: {}".format(get_message(pre, pre_msg))
# AssertionError: Pre-condition failed: Onoes !