output formatting for classes

I'd like to get output formatting for my own classes that mimics the
built-in output formatting. For example,
[color=blue][color=green][color=darkred]
>>> x = 4.54
>>> print "%4.2f" % x[/color][/color][/color]
4.54

In other words, if I substitute a class instance for "x" above, I'd
like to make the format string apply to an element or elements of the
instance. Can I somehow overload the "%" operator for that? Thanks.

On an unrelated matter, I think the complex class in Python is too
complex, so I plan to clean it up and implement it right. (just
kidding, folks!)

re: output formatting for classes

Russ wrote:[color=blue]
> I'd like to get output formatting for my own classes that mimics the
> built-in output formatting. For example,
>
>[color=green][color=darkred]
>>>>x = 4.54
>>>>print "%4.2f" % x[/color][/color]
>
> 4.54
>
> In other words, if I substitute a class instance for "x" above, I'd
> like to make the format string apply to an element or elements of the
> instance. Can I somehow overload the "%" operator for that? Thanks.
>
> On an unrelated matter, I think the complex class in Python is too
> complex, so I plan to clean it up and implement it right. (just
> kidding, folks!)[/color]

yeah, i miss some things in complex implementation
for example c=complex()
c.abs = 2**0.5
c.angle = pi/2

should result in 1+1j :)

or c=complex(1,1)
print c.abs # should print 2**0.5
print c.angle # should print pi%2

i think one can implement it with properties

but to your question ...
[color=blue][color=green][color=darkred]
>>> class X(object):[/color][/color][/color]
.... def __float__(self):
.... return 1.0
.... def __long__(self):
.... return 10l
.... def __int__(self):
.... return 20
.... def __repr__(self):
.... return "i am"
.... def __str__(self):
.... return "I AM"
.... def __complex__(self):
.... return 1+1j
....[color=blue][color=green][color=darkred]
>>> x=X()
>>> int(x)[/color][/color][/color]
20[color=blue][color=green][color=darkred]
>>> long(x)[/color][/color][/color]
10L[color=blue][color=green][color=darkred]
>>> float(x)[/color][/color][/color]
1.0[color=blue][color=green][color=darkred]
>>> str(x)[/color][/color][/color]
'I AM'[color=blue][color=green][color=darkred]
>>> repr(x)[/color][/color][/color]
'i am'[color=blue][color=green][color=darkred]
>>> print "%s -- %r" % (x,x)[/color][/color][/color]
I AM -- i am[color=blue][color=green][color=darkred]
>>> complex(x)[/color][/color][/color]
(1+1j)[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

re: output formatting for classes

On Fri, 10 Mar 2006 02:19:10 +0100, Schüle Daniel wrote:
[color=blue]
> yeah, i miss some things in complex implementation
> for example c=complex()
> c.abs = 2**0.5
> c.angle = pi/2
>
> should result in 1+1j :)[/color]

Smiley noted, but consider:

c = complex()
=> what is the value of c here?

c.abs = 2**0.5
=> what is c's value now?

c.angle = pi/2
=> now c has the value 1+1j

Objects with indeterminate values are rarely a good idea.

A better way would be for complex numbers to take a constructor that can
take arguments in either Cartesian or polar form. So, hypothetically, the
following would all be equivalent:

1+1j
complex(1,1)
complex(real=1, img=1)
complex(len=2**0.5, theta=pi/2)

Another alternative would be a function to construct polar form complex
numbers. It could be a plain function or a static method:

cmath.polar(2**0.5, pi/2) => 1+1j
complex.polar(2**0.5, pi/2) => 1+1j



--
Steven.

re: output formatting for classes

Steven D'Aprano wrote:[color=blue]
> On Fri, 10 Mar 2006 02:19:10 +0100, Schüle Daniel wrote:
>
>[color=green]
>>yeah, i miss some things in complex implementation
>>for example c=complex()
>>c.abs = 2**0.5
>>c.angle = pi/2
>>
>>should result in 1+1j :)[/color]
>
>
> Smiley noted, but consider:
>
> c = complex()
> => what is the value of c here?[/color]

default value is 0, for complex number that means
real = 0, imag = 0
is the same as
c.abs=0, c.angle=0

ok mathematically c.angle can be of arbitrary value
but defaulting it to zero is very handy
c = complex()
c.abs = 10
yields 10+0j

c=complex()
c.real = 2
c.imag = 2
c.abs = 50**0.5 # angle remains, length changed
yields 5+5j
c.angle = 0
yields 50**0.5 + 0j
[color=blue]
> c.abs = 2**0.5
> => what is c's value now?[/color]

c.abs = 2**0.5
c.angle = 0
[color=blue]
>
> c.angle = pi/2
> => now c has the value 1+1j
>
> Objects with indeterminate values are rarely a good idea.[/color]

IMHO it's perfectly consistent with[color=blue][color=green][color=darkred]
>>> int()[/color][/color][/color]
0[color=blue][color=green][color=darkred]
>>> long()[/color][/color][/color]
0L[color=blue][color=green][color=darkred]
>>> float()[/color][/color][/color]
0.0[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]
complex()[color=blue][color=green][color=darkred]
>>> complex()[/color][/color][/color]
0j[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

but extending complex with default angle=0


[color=blue]
> A better way would be for complex numbers to take a constructor that can
> take arguments in either Cartesian or polar form. So, hypothetically, the
> following would all be equivalent:
>
> 1+1j
> complex(1,1)
> complex(real=1, img=1)
> complex(len=2**0.5, theta=pi/2)[/color]

ack
but after the creation of complex number one will have to
do all the transformations in another coord. system manually
[color=blue]
> Another alternative would be a function to construct polar form complex
> numbers. It could be a plain function or a static method:
>
> cmath.polar(2**0.5, pi/2) => 1+1j
> complex.polar(2**0.5, pi/2) => 1+1j[/color]

maybe adding

c=complex.from_polar((length,angle))
d=complex.to_polar(c)
d == (length, angle)
True

would be sufficient, but I would prefer the other version

Regards