Re: how to build a street with more than 1 house ?
On Sat, 07 Jun 2008 00:45:07 +0200, Stef Mientki [EMAIL PROTECTED] wrote: hello, In the code below, I can build a large street like this: large_street = house * 25 but not a small street. like this: small_street = 5 * house Why is this different ? Because you're multiplying on the left in one case and on the right in the other. You realize that house*5 should work, right? And more interesting, how do I get the right results ? thanks, Stef Mientki class type_house ( object ) : def __init__( self, front_doors = 1 ) : self.front_doors = front_doors def __mul__ ( self, b ) : return type_house ( self.front_doors * b ) The reason house*25 works is that the __mul__ method says what it should be. If you want 5*house to work you need a __rmul__. Nothing in Python automatically makes a*b the same as b*a; when it sees 5*house first it checks 5 to see whether it knows whether it knows what 5*house should be (no, 5 never heard of this house thing), then it checks house to see if it knows what 5*house should be (no, house has no __rmul__). The simplest thing is just to define __rmul__ to make multiplication commuttative: def __rmul__(self, b): Defines what b*self should return. return self*b Now 5*house calls __rmul__, which returns house*5. That in turn calls __mul__, which returns what you want. And some day when you modify __mul__ (in a subclass?) you won't need to worry about making the same change to __rmul__. house = type_house () large_street = house * 25 print large_street.front_doors small_street = 5 * house print small_street.front_doors David C. Ullrich -- http://mail.python.org/mailman/listinfo/python-list
Re: how to build a street with more than 1 house ?
thanks guys, David C. Ullrich wrote: On Sat, 07 Jun 2008 00:45:07 +0200, Stef Mientki [EMAIL PROTECTED] wrote: hello, In the code below, I can build a large street like this: large_street = house * 25 but not a small street. like this: small_street = 5 * house Why is this different ? Because you're multiplying on the left in one case and on the right in the other. You realize that house*5 should work, right? yes. And more interesting, how do I get the right results ? thanks, Stef Mientki class type_house ( object ) : def __init__( self, front_doors = 1 ) : self.front_doors = front_doors def __mul__ ( self, b ) : return type_house ( self.front_doors * b ) The reason house*25 works is that the __mul__ method says what it should be. If you want 5*house to work you need a __rmul__. Nothing in Python automatically makes a*b the same as b*a; when it sees 5*house first it checks 5 to see whether it knows whether it knows what 5*house should be (no, 5 never heard of this house thing), then it checks house to see if it knows what 5*house should be (no, house has no __rmul__). The simplest thing is just to define __rmul__ to make multiplication commuttative: Aha, now a bell begins to ring ... def __rmul__(self, b): Defines what b*self should return. return self*b this works indeed ... Now 5*house calls __rmul__, which returns house*5. That in turn calls __mul__, which returns what you want. And some day when you modify __mul__ (in a subclass?) you won't need to worry about making the same change to __rmul__. thanks for the clear explanation. cheers, Stef house = type_house () large_street = house * 25 print large_street.front_doors small_street = 5 * house print small_street.front_doors David C. Ullrich -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
how to build a street with more than 1 house ?
hello, In the code below, I can build a large street like this: large_street = house * 25 but not a small street. like this: small_street = 5 * house Why is this different ? And more interesting, how do I get the right results ? thanks, Stef Mientki class type_house ( object ) : def __init__( self, front_doors = 1 ) : self.front_doors = front_doors def __mul__ ( self, b ) : return type_house ( self.front_doors * b ) house = type_house () large_street = house * 25 print large_street.front_doors small_street = 5 * house print small_street.front_doors -- http://mail.python.org/mailman/listinfo/python-list
Re: how to build a street with more than 1 house ?
On Jun 6, 2008, at 6:45 PM, Stef Mientki wrote: hello, In the code below, I can build a large street like this: large_street = house * 25 but not a small street. like this: small_street = 5 * house This calls the int.__mul__() code i believe. Why is this different ? And more interesting, how do I get the right results ? If memory serves me right look into __rmul__(). Cheers Tommy -- http://mail.python.org/mailman/listinfo/python-list
