Lazy evaluated


H

Habibutsu

For example, we have following code:

01| def foo():
02| return 1
03|
04| value = foo()
05|
06| if value == 1:
07| print value,"- equal 1"
08|
09| if isinstance(value, int):
10| print value,"- is int"
11| else:
12| print value,"- is not int"

Task is to create lazy evaluation for function 'foo'.
For decision this task we create special function 'lazy' which turn
original function into a lazy evaluated function by means of creating
proxy object that evaluated value if needed. We add following code:

01| def lazy(func):
02|
03| class ProxyLazy(object):
04| _result_value = None
05|
06| @property
07| def result_value(self):
08| if self._result_value is not None:
09| return self._result_value
10| self._result_value = self.func(*self.args, **self.kw)
11| return self._result_value
12|
13| def __str__(self):
14| return str(self.result_value)
15|
16| def __cmp__(self, other):
17| return cmp(self.result_value, other)
18|
19| # and other __unicode__, __eq__, __ne__ and so on
20|
21| def wrapper(*args, **kw):
22| proxy = ProxyLazy()
23| proxy.func = func
24| proxy.args = args
25| proxy.kw = kw
26| return proxy
27|
28| return wrapper
29|
30| lazy_foo = lazy(foo)
31| value = lazy_foo()

Unfortunately, this method not allow to use 'isinstance' for check type.
We can create other variant of function 'lazy' is 'lazy_promise' in
which additional parameter will be passed with type of result value.
After we can create 'LazyMetaClass' with helping of which will be
created 'ProxyLazy'

01| def lazy_promise(func, resultclass):
02|
03| class LazyMetaClass(type):
04| def __new__(cls, name, bases, attrs):
05| return super(LazyMetaClass, cls).__new__(cls, name,
(resultclass,), attrs)
06|
07| class ProxyLazy(object):
08| __metaclass__ = LazyMetaClass
....
35| lazy_foo = lazy_promise(foo, int)
36| value = lazy_foo()

And everything seems to work, but appear other questions. If original
function return different types - what to do in this case? Where i am
wrong? What other way to do that. Was no idea to create keyword 'lazy'
in Python?

Thanks
 
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S

Steven D'Aprano

For example, we have following code:

01| def foo():
02| return 1
03|
04| value = foo()
05|
06| if value == 1:
07| print value,"- equal 1"
08|
09| if isinstance(value, int):
10| print value,"- is int"
11| else:
12| print value,"- is not int"

Task is to create lazy evaluation for function 'foo'. For decision this
task we create special function 'lazy' which turn original function into
a lazy evaluated function by means of creating proxy object that
evaluated value if needed. We add following code:


01| def lazy(func):
02|
03| class ProxyLazy(object):
04| _result_value = None
05|
06| @property
07| def result_value(self):
[...]


It is often useful for the reader to copy and paste code blocks like this
into the interactive interpreter. It is more friendly and useful if you
paste it in a format that makes that easy (no prompts, no line numbers,
no blank lines inside classes or functions). Otherwise the reader has to
copy your code, paste it into an editor, strip out the line numbers and
blank lines, copy and paste it into the interpreter, and THEN they can
finally test your code and see what it does.

30| lazy_foo = lazy(foo)
31| value = lazy_foo()

Unfortunately, this method not allow to use 'isinstance' for check type.

Correct. And so it should not, because it value is not an int, it is a
ProxyLazy object. You cannot use a ProxyLazy object where an int is
expected:

py> value + 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'ProxyLazy' and 'int'


so it is inappropriate to claim that value is an int.

I must admit, I don't understand the value of this "lazy proxy" type you
have created. You get a proxy like this:

value = lazy(foo)() # get a lazy proxy
# much later
value = value.func() # now safe to use as an int
print(value + 1)

What benefit does the lazy proxy give? Why not just do this?


value = foo() # functions are already a lazy proxy
# much later
value = value() # now safe to use as an int
print(value + 1)


[...]
And everything seems to work, but appear other questions. If original
function return different types - what to do in this case? Where i am
wrong? What other way to do that. Was no idea to create keyword 'lazy'
in Python?

Why should it be a keyword? Python has very few keywords, and most of
them are for programming flow control, like "if", "else", "for", "while",
"pass", etc.
 
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H

Habibutsu

For example, we have following code:

01| def foo():
02| return 1
03|
04| value = foo()
05|
06| if value == 1:
07| print value,"- equal 1"
08|
09| if isinstance(value, int):
10| print value,"- is int"
11| else:
12| print value,"- is not int"

Task is to create lazy evaluation for function 'foo'. For decision this
task we create special function 'lazy' which turn original function into
a lazy evaluated function by means of creating proxy object that
evaluated value if needed. We add following code:


01| def lazy(func):
02|
03| class ProxyLazy(object):
04| _result_value = None
05|
06| @property
07| def result_value(self):
[...]


It is often useful for the reader to copy and paste code blocks like this
into the interactive interpreter. It is more friendly and useful if you
paste it in a format that makes that easy (no prompts, no line numbers,
no blank lines inside classes or functions). Otherwise the reader has to
copy your code, paste it into an editor, strip out the line numbers and
blank lines, copy and paste it into the interpreter, and THEN they can
finally test your code and see what it does.
Previously I not to do it, yesterday decide to experiment. It seemed to
me more readable if to see in the email-clent. Sorry, I will keep in
mind in the future.
Correct. And so it should not, because it value is not an int, it is a
ProxyLazy object. You cannot use a ProxyLazy object where an int is
expected:

py> value + 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'ProxyLazy' and 'int'
Why not?

def __add__(self, other):
return self._result_value + other

and then
2

so it is inappropriate to claim that value is an int.

I can overload all arithmetic operators and get almost really int value,
but I don't like to write many trivial code.
I must admit, I don't understand the value of this "lazy proxy" type you
have created. You get a proxy like this:

value = lazy(foo)() # get a lazy proxy
# much later
value = value.func() # now safe to use as an int
print(value + 1)

What benefit does the lazy proxy give? Why not just do this?
Because some people like to use "list comprehensions" and other not,
some people like labmda and other not. Advantage is that I can write
more expressive code and use different strategies for different cases.
"value = value.func()" is looks sad while I just need the value.
value = foo() # functions are already a lazy proxy
# much later
value = value() # now safe to use as an int
print(value + 1)


[...]
And everything seems to work, but appear other questions. If original
function return different types - what to do in this case? Where i am
wrong? What other way to do that. Was no idea to create keyword 'lazy'
in Python?
Why should it be a keyword? Python has very few keywords, and most of
them are for programming flow control, like "if", "else", "for", "while",
"pass", etc.
Question not in why I can't use exist tools - I can. If i can write code
without classes that it does not mean that classes need to delete from
language. Question why in language not present this ability?

Thanks
 

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