157 lines
3.4 KiB
Markdown
157 lines
3.4 KiB
Markdown
# Exercise 7.2 - Solution
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## (a) Copying Metadata
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```python
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# logcall.py
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from functools import wraps
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def logged(func):
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print('Adding logging to', func.__name__)
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@wraps(func)
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def wrapper(*args,**kwargs):
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print('Calling', func.__name__)
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return func(*args,**kwargs)
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return wrapper
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```
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## (b) Decorators with arguments
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```python
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# logcall.py
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from functools import wraps
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...
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def logformat(fmt):
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def logged(func):
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print('Adding logging to', func.__name__)
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@wraps(func)
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def wrapper(*args, **kwargs):
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print(fmt.format(func=func))
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return func(*args, **kwargs)
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return wrapper
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return logged
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```
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The earlier `@logged` decorator can be rewritten as follows:
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```python
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logged = logformat('Calling {func.__name__}')
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```
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## (c) Decorators and methods
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You can get the code to work if you interchange the order of the
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decorators. For example:
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```python
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from logcall import logged
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class Spam:
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@logged
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def instance_method(self):
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pass
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@classmethod
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@logged
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def class_method(cls):
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pass
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@staticmethod
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@logged
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def static_method():
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pass
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@property
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@logged
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def property_method(self):
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pass
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```
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Ponder why it doesn't work in the original order. Is there any way to make
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the `@logged` decorator work regardless of the order in which its applied?
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## (d) Validation (Redux)
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```python
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# validate.py
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...
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from inspect import signature
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from functools import wraps
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def validated(func):
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sig = signature(func)
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# Gather the function annotations
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annotations = dict(func.__annotations__)
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# Get the return annotation (if any)
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retcheck = annotations.pop('return', None)
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@wraps(func)
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def wrapper(*args, **kwargs):
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bound = sig.bind(*args, **kwargs)
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errors = []
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# Enforce argument checks
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for name, validator in annotations.items():
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try:
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validator.check(bound.arguments[name])
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except Exception as e:
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errors.append(f' {name}: {e}')
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if errors:
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raise TypeError('Bad Arguments\n' + '\n'.join(errors))
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result = func(*args, **kwargs)
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# Enforce return check (if any)
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if retcheck:
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try:
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retcheck.check(result)
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except Exception as e:
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raise TypeError(f'Bad return: {e}') from None
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return result
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return wrapper
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def enforce(**annotations):
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retcheck = annotations.pop('return_', None)
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def decorate(func):
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sig = signature(func)
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@wraps(func)
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def wrapper(*args, **kwargs):
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bound = sig.bind(*args, **kwargs)
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errors = []
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# Enforce argument checks
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for name, validator in annotations.items():
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try:
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validator.check(bound.arguments[name])
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except Exception as e:
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errors.append(f' {name}: {e}')
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if errors:
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raise TypeError('Bad Arguments\n' + '\n'.join(errors))
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result = func(*args, **kwargs)
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if retcheck:
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try:
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retcheck.check(result)
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except Exception as e:
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raise TypeError(f'Bad return: {e}') from None
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return result
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return wrapper
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return decorate
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```
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[Back](ex7_2.md)
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