tolerance

A function decorator which makes a function tolerant (the function fail silently).

Project description

Author: Alisue <lambdalisue@hashnote.net>
Supported python versions: 2.6, 2.7, 3.2, 3.3, 3.4

Do you often write the fail silent codes like below?

try:
    # do what ever you need...
    return "foo"
except:
    # fail silently
    return ""

This kind of codes are often found in Django projects or programs which should not raise any exceptions in product mode.

tolerance is a function decorator to make a tolerant function; a function which does not raise any exceptions even there are exceptions. This concept is quite useful for making stable product or prefer_int types of code described in Usage section.

Check online documentation for more details.

Features

Convert a function to a tolerant function
The decorated function returns substitute (Default is None) when it is not callable. The function returns a “returned value” from substitute function when it is callable.
Ignoreing exceptions can be specified as a exception class list with exceptions argument.
When fail_silently=False is passed to the decorated function, the function does not ignore exceptions (the argument name can be changed with making switch function via argument_switch_generator function).

Installation

Use pip like:

$ pip install tolerance

Usage

Assume that you need a function which convert a string to an integer when it is possible. Without tolerance, you need to write a code like below

>>> # without tolerance
>>> def prefer_int_withot_tolerance(x):
...     try:
...         return int(x)
...     except:
...         # fail silently
...         return x
>>> prefer_int_withot_tolerance(0)
0
>>> prefer_int_withot_tolerance('0')
0
>>> prefer_int_withot_tolerance('zero')
'zero'

However, with tolerance, you just need to write a single line code like

>>> from tolerance import tolerate
>>> prefer_int = tolerate(lambda x: x)(int)
>>> prefer_int(0)
0
>>> prefer_int('0')
0
>>> prefer_int('zero')
'zero'

Or you can use tolerate as a function decorator described in PEP-318

>>> from tolerance import tolerate
>>> @tolerate(lambda x: x)
... def prefer_int_318(x):
...     return int(x)
>>> prefer_int_318(0)
0
>>> prefer_int_318('0')
0
>>> prefer_int_318('zero')
'zero'

The example codes above specify substitute argument of tolerate function to specify the returning value when the function has failed ( lambda x: x part). tolerate function takes several arguments to configure the function behavior. These arguments are explained in Case study and detailed in API documentation.

Change log

Version 0.1.0

Initial development
Manually tested with Python 2.4, 2.5, 2.7, 3.2, 3.3

Version 0.1.1

switch shortcut feature is added
Drop off supporting Python 2.4 and 2.5
Support Python 3.2 and 3.3 via 2to3
Use tox for testing

Case study

Q. How can I return the default value when the function fail?

Use substitute argument to specify the default value like

>>> from tolerance import tolerate
>>> @tolerate(substitute='foo')
... def raise_exception():
...     raise Exception
>>> raise_exception()
'foo'

Q. How can I change the default value depends on passed arguments?

Specify substitute argument as a function

>>> from tolerance import tolerate
>>> def substitute_function(*args, **kwargs):
...     # do what ever you need, this example simply return 1st argument
...     return args[0]
>>> @tolerate(substitute=substitute_function)
... def raise_exception(*args):
...     raise Exception
>>> raise_exception('bar', 'hoge')
'bar'

Q. How can I make the function to ignore only several exceptions?

Use exceptions argument to specify exceptions which will be ignored.

>>> from tolerance import tolerate
>>> exceptions_ignored = (
...     AttributeError,
...     ValueError,
... )
>>> @tolerate(exceptions=exceptions_ignored)
... def raise_exception(x):
...     if x == 0:
...         raise AttributeError
...     elif x == 1:
...         raise ValueError
...     else:
...         raise KeyError
>>> raise_exception(0) is None
True
>>> raise_exception(1) is None
True
>>> raise_exception(2)
Traceback (most recent call last):
    ...
KeyError

Q. How can I disable ignoreing exceptions in the decorated function?

Pass fail_silently=False to the decorated function.

>>> from tolerance import tolerate
>>> @tolerate()
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
True
>>> raise_exception(fail_silently=False)
Traceback (most recent call last):
    ...
KeyError

You can change the attribute name with specifing new switch function. It will be explained below.

Q. How can I disable ignoreing exceptions globally?

Set tolerate.disabled = True to disable tolerance globally.

>>> from tolerance import tolerate
>>> @tolerate()
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
True
>>> tolerate.disabled = True
>>> raise_exception()
Traceback (most recent call last):
    ...
KeyError
>>> # rollback
>>> tolerate.disabled = False

Q. How can I disable ignoreing exceptions in complex mannar?

Use switch argument to specify switch function.

>>> from tolerance import tolerate
>>> DEBUG = False
>>> def switch_function(*args, **kwargs):
...     # do what ever you need, this sample check kwargs and DEBUG
...     # remove 'fail_silently' attribute and store
...     fail_silently = kwargs.pop('fail_silently', True)
...     if DEBUG or not fail_silently:
...         # do not ignore exceptions. note that kwargs which does not
...         # have 'fail_silently' is returned back.
...         return False, args, kwargs
...     # do ignore exceptions. note that kwargs which does not have
...     # 'fail_silently' is returned back.
...     return True, args, kwargs
>>> @tolerate(switch=switch_function)
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
True
>>> raise_exception(fail_silently=False)
Traceback (most recent call last):
    ...
KeyError
>>> DEBUG = True
>>> raise_exception()
Traceback (most recent call last):
    ...
KeyError

Q. I just want to change the attribute name, making switch function is too complicated

Use argument_switch_generator to make switch function.

>>> from tolerance import tolerate
>>> from tolerance import argument_switch_generator
>>> switch_function = argument_switch_generator('quiet')
>>> @tolerate(switch=switch_function)
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
True
>>> # you can use `quiet=False` instead of `fail_silently`
>>> raise_exception(quiet=False)
Traceback (most recent call last):
    ...
KeyError
>>> # raise_exception does not know fail_silently so ignore
>>> raise_exception(fail_silently=False) is None
True
>>> #
>>> # From Version 0.1.1
>>> #
>>> @tolerate(switch='quiet')
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
True
>>> raise_exception(quiet=False)
Traceback (most recent call last):
    ...
KeyError
>>> raise_exception(fail_silently=False) is None
True

Q. I want to make the function ignoreing exceptions only when fail_silently=True is passed

Use default argument to tell argument_switch_generator function

>>> from tolerance import tolerate
>>> from tolerance import argument_switch_generator
>>> switch_function = argument_switch_generator('fail_silently', default=False)
>>> @tolerate(switch=switch_function)
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
Traceback (most recent call last):
    ...
KeyError
>>> raise_exception(fail_silently=True) is None
True
>>> #
>>> # From Version 0.1.1
>>> #
>>> @tolerate(switch=[None, False])
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
Traceback (most recent call last):
    ...
KeyError
>>> @tolerate(switch={'default': False})
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
Traceback (most recent call last):
    ...
KeyError

Q. I want to disable the ignoreing exceptions when verbose=False is passed

Use reverse argument to tell argument_switch_generator function

>>> from tolerance import tolerate
>>> from tolerance import argument_switch_generator
>>> switch_function = argument_switch_generator('verbose', reverse=True)
>>> @tolerate(switch=switch_function)
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
True
>>> raise_exception(verbose=True)
Traceback (most recent call last):
    ...
KeyError
>>> #
>>> # From Version 0.1.1
>>> #
>>> @tolerate(switch={'argument_name': 'verbose', 'reverse': True})
... def raise_exception():
...     raise KeyError
>>> raise_exception() is None
True
>>> raise_exception(verbose=True)
Traceback (most recent call last):
    ...
KeyError

Q. I want to use fail_silently argument even in decorated function

Use keep argument to tell argument_switch_generator function

>>> from tolerance import tolerate
>>> from tolerance import argument_switch_generator
>>> switch_function = argument_switch_generator('fail_silently', keep=True)
>>> @tolerate(switch=switch_function)
... def raise_exception(**kwargs):
...     if 'fail_silently' in kwargs:
...         raise KeyError
...     return 'Failed!'
>>> raise_exception(fail_silently=True) is None
True
>>> raise_exception(fail_silently=False)
Traceback (most recent call last):
    ...
KeyError
>>> #
>>> # From Version 0.1.1
>>> #
>>> @tolerate(switch={'keep': True})
... def raise_exception(**kwargs):
...     if 'fail_silently' in kwargs:
...         raise KeyError
...     return 'Failed!'
>>> raise_exception(fail_silently=True) is None
True
>>> raise_exception(fail_silently=False)
Traceback (most recent call last):
    ...
KeyError

Project details

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This version

0.1.2

Jul 21, 2014

0.1.1

Jan 21, 2014

0.1.0

Jan 20, 2014

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tolerance 0.1.2

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Project description

Features

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Usage

Change log

Case study

Q. How can I return the default value when the function fail?

Q. How can I change the default value depends on passed arguments?

Q. How can I make the function to ignore only several exceptions?

Q. How can I disable ignoreing exceptions in the decorated function?

Q. How can I disable ignoreing exceptions globally?

Q. How can I disable ignoreing exceptions in complex mannar?

Q. I just want to change the attribute name, making switch function is too complicated

Q. I want to make the function ignoreing exceptions only when fail_silently=True is passed

Q. I want to disable the ignoreing exceptions when verbose=False is passed

Q. I want to use fail_silently argument even in decorated function

Project details

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Release history Release notifications | RSS feed

Download files

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