On 03/10/2017 15:39, Ian Kelly wrote:
On Tue, Oct 3, 2017 at 4:41 AM, Steve D'Aprano
<steve+pyt...@pearwood.info> wrote:
On Tue, 3 Oct 2017 06:51 am, Bill wrote:
Can you inspire me with a good decorator problem (standard homework
exercise-level will be fine)?
Here is a nice even dozen problems for you. Please ask for clarification if any
are unclear.
(1) Write a decorator which simply prints a descriptive message and the name of
the decorated function once, when the function is first decorated.
E.g. if you write:
@decorate
def spam(x):
return x + 1 # for example
print(spam(1))
print(spam(2))
Python should print:
Decorating function spam.
2
3
Note: "spam" must not be hard-coded, it must be taken from the function being
decorated. (Hint: all functions have their name available as func.__name__.)
(2) Modify the decorator from (1) so that calling the wrapped function also
print a descriptive message such as "Calling function spam". The expected
output will be:
Decorating function spam.
Calling function spam.
2
Calling function spam.
3
(3) Write a decorator that checks that the decorated function's first argument
is a non-empty string, raising an appropriate exception if it is not, and lets
through any other arguments unchanged.
(4) Same as above, except the first argument is automatically stripped of
leading and trailing whitespace and forced to uppercase.
(5) Write a decorator which injects the argument 10 into the list of arguments
received by the wrapped function. E.g. if you write:
@inject
def add(a, b):
return a + b
@inject
def sub(a, b):
return a - b
print(add(5), sub(5))
Python should print "15 5". (And *not* "15 -5".)
(6) [ADVANCED] Modify the decorator in (5) so that it takes an argument telling
it what value to inject into the list of arguments:
@inject(99)
def sub(a, b):
return a - b
print(sub(5))
will now print "94".
(7) Write a decorator which checks the decorated function's two arguments are
given smallest first, swapping them around if needed.
(8) Write a decorator which prints the name of the wrapped function, its
arguments, and the time, each time the wrapped function is called.
(9) [ADVANCED] Modify the decorator from (8) to take an argument specifying the
path to a file, and use the logging module to log the details to that file
instead of printing them.
(10) Write a decorator which adds an "cache" attribute initialised to an empty
dictionary to the decorated function.
(11) Write a decorator which wraps a class (not function!), and adds a "help"
method to the class which prints a message as shown below. For example:
@addhelp
class Spam:
pass
@addhelp
class Eggs:
pass
x = Spam()
x.help()
y = Eggs()
y.help()
will print:
See http://example.com/Spam
See http://example.com/Eggs
(Hint: classes also have a __name__ attribute.)
(12) [ADVANCED] Write a decorator which wraps a class, and applies the decorator
from (10) above to each non-dunder¹ method in the class. That is, after:
@addcaches
class MyClass:
def foo(self):
pass
def bar(self):
pass
print(MyClass.foo.cache, MyClass.bar.cache)
should print "{} {}".
¹ Remember that dunder methods are those that start with two leading and
trailing underscores: "Double UNDERscore" methods.
[Sorry can't see Steve's original post.]
Does all this advanced stuff (which I don't understand and which doesn't
look very appealing either; hopefully I will never come across such
code) still count as programming?
It seems to me the equivalent of an advanced driving course teaching you
how to customise your car rather than involving any actual driving.
--
bartc
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