Re: Method default argument whose type is the class not yet defined
On 11 November 2012 02:47, Chris Angelico ros...@gmail.com wrote: On Sun, Nov 11, 2012 at 1:43 PM, Ian Kelly ian.g.ke...@gmail.com wrote: On Sat, Nov 10, 2012 at 7:13 PM, Chris Angelico ros...@gmail.com wrote: I would not assume that. The origin is a point, just like any other. With a Line class, you could deem a zero-length line to be like a zero-element list, but Point(0,0) is more like the tuple (0,0) which is definitely True. It's more like the number 0 than the tuple (0,0). 0 is the origin on a 1-dimensional number line. (0,0) is the origin on a 2-dimensional number plane. In fact, it might be pointed out that Point(0, 0) is a generalization of 0+0j, which is equal to 0. Ah, good point. In any case, though, it'd be an utterly inconsequential bug. You were right the first time, Chris. A point that happens to coincide with the arbitrarily chosen origin is no more truthy or falsey than any other. A vector of length 0 on the other hand is a very different beast. The significance of zero in real algebra is not that it is the origin but rather that it is the additive and multiplicative zero: a + 0 = a for any real number a a * 0 = 0 for any real number a The same is true for a vector v0, of length 0: v + v0 = v for any vector v a * v0 = v0 for any scalar a There is however no meaningful sense in which points (as opposed to vectors) can be added to each other or multiplied by anything, so there is no zero point. The relationship between points and vectors is analogous to the relationship between datetimes and timedeltas. Having Vector(0, 0) evaluate to False is analogous to having timedelta(0) evaluate to False and is entirely sensible. Having Point(0, 0) evaluate to False is precisely the same conceptual folly that sees midnight evaluate as False. Oscar -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On Sun, 11 Nov 2012 14:21:19 +, Oscar Benjamin wrote: On 11 November 2012 02:47, Chris Angelico ros...@gmail.com wrote: On Sun, Nov 11, 2012 at 1:43 PM, Ian Kelly ian.g.ke...@gmail.com wrote: On Sat, Nov 10, 2012 at 7:13 PM, Chris Angelico ros...@gmail.com wrote: I would not assume that. The origin is a point, just like any other. With a Line class, you could deem a zero-length line to be like a zero-element list, but Point(0,0) is more like the tuple (0,0) which is definitely True. Don't conflate the set of all tuples of arbitrary length with points, which have fixed length. Points are not *sequences* -- in Python, we treat tuples as sequences first and records second, because that is a useful thing to do. (But it is not the only useful thing to do: if we treated them as records first and sequences second, we might want to say that a tuple t was falsey if all the fields in t were falsey.) In the case of a Point class, a Point is definitely not a sequence. That we put the x-coordinate first and the y-coordinate second is a mere convention, like writing left to right. The mathematical properties of points do not depend on the x-coordinate coming first. Since points should not be treated as sequences, the requirement that non-empty sequences be treated as truthy is irrelevant. It's more like the number 0 than the tuple (0,0). 0 is the origin on a 1-dimensional number line. (0,0) is the origin on a 2-dimensional number plane. In fact, it might be pointed out that Point(0, 0) is a generalization of 0+0j, which is equal to 0. Ah, good point. In any case, though, it'd be an utterly inconsequential bug. You were right the first time, Chris. A point that happens to coincide with the arbitrarily chosen origin is no more truthy or falsey than any other. A vector of length 0 on the other hand is a very different beast. Nonsense. The length and direction of a vector is relative to the origin. If the origin is arbitrary, as you claim, then so is the length of the vector. Just because we can perform vector transformations on the plane to move the origin to some point other that (0,0) doesn't make (0,0) an arbitrarily chosen origin. It is no more arbitrary than 0 as the origin of the real number line. And yes, we can perform 1D vector transformations on the real number line too. Here's a version of range that sets the origin to 42, not 0: def myrange(start, end=None, step=1): if end is None: start = 42 return range(start, end, step) Nevertheless, there really is something special about the point 0 on the real number line, the point (0,0) on the complex number plane, the point (0,0,0) in the 3D space, (0,0,0,0) in 4D space, etc. It is not just an arbitrary convention that we set the origin to 0. In other words: to the extent that your arguments that zero-vectors are special are correct, the same applies to zero-points, since vectors are defined as a magnitude and direction *from the origin*. To put it yet another way: The complex number a+bj is equivalent to the 2D point (a, b) which is equivalent to the 2D vector [a, b]. If (0, 0) shouldn't be considered falsey, neither should [0, 0]. The significance of zero in real algebra is not that it is the origin but rather that it is the additive and multiplicative zero: a + 0 = a for any real number a a * 0 = 0 for any real number a I'm not sure what you mean by additive and multiplicative zero, you appear to be conflating two different properties here. 0 is the additive *identity*, but 1 is the multiplicative identity: a + 0 = a a * 1 = a for any real number a. If the RHS must be zero, then there is a unique multiplicative zero, but no unique additive zero: a * 0 = 0 for any real number a a + -a = 0 for any real number a The same is true for a vector v0, of length 0: v + v0 = v for any vector v a * v0 = v0 for any scalar a Well that's a bogus analogy. Since you're talking about the domain of vectors, the relevant identify for the second line should be: v * v0 = v0 for any vector v except that doesn't work, since vector algebra doesn't define a vector multiplication operator.[1] It does define multiplication between a vector and a scalar, which represents a scale transformation. There is however no meaningful sense in which points (as opposed to vectors) can be added to each other or multiplied by anything, so there is no zero point. I think that the great mathematician Carl Gauss would have something to say about that. Points in the plane are equivalent to complex numbers, and you can certainly add and multiply complex numbers. Adding two points is equivalent to a translation; multiplication of a scalar with a point is equivalent to a scale transformation. Multiplying two points is equivalent to complex multiplication, which is a scale + a rotation. Oh look, that's exactly the same geometric interpretation as for vectors. Hardly surprising,
Re: Method default argument whose type is the class not yet defined
On 11 November 2012 22:31, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: On Sun, 11 Nov 2012 14:21:19 +, Oscar Benjamin wrote: On 11 November 2012 02:47, Chris Angelico ros...@gmail.com wrote: On Sun, Nov 11, 2012 at 1:43 PM, Ian Kelly ian.g.ke...@gmail.com wrote: On Sat, Nov 10, 2012 at 7:13 PM, Chris Angelico ros...@gmail.com wrote: I would not assume that. The origin is a point, just like any other. With a Line class, you could deem a zero-length line to be like a zero-element list, but Point(0,0) is more like the tuple (0,0) which is definitely True. It's more like the number 0 than the tuple (0,0). 0 is the origin on a 1-dimensional number line. (0,0) is the origin on a 2-dimensional number plane. In fact, it might be pointed out that Point(0, 0) is a generalization of 0+0j, which is equal to 0. Ah, good point. In any case, though, it'd be an utterly inconsequential bug. You were right the first time, Chris. A point that happens to coincide with the arbitrarily chosen origin is no more truthy or falsey than any other. A vector of length 0 on the other hand is a very different beast. Nonsense. The length and direction of a vector is relative to the origin. If the origin is arbitrary, as you claim, then so is the length of the vector. Wrong on all counts. Neither the length not the direction os a vector are relative to any origin. When we choose to express a vector in Cartesian components our representation assumes an orientation for the axes of the coordinate system. Even in this sense, though, the origin itself does not affect the components of the vector. I have spent a fair few hours in the past few weeks persuading teenaged Engineering students to maintain a clear distinction between points, vectors and lines. One of the ways that I distinguish vectors from points is to say that a vector is like an arrow but its base has no particular position. A point on the other hand is quite simply a position. Given an origin (an arbitrarily chosen point) we can specify another point using a position vector: a vector from the origin to the point in question. Just because we can perform vector transformations on the plane to move the origin to some point other that (0,0) doesn't make (0,0) an arbitrarily chosen origin. It is no more arbitrary than 0 as the origin of the real number line. (0, 0) are the coordinates of the origin *relative to itself*. Had we chosen a different origin, the point that was previously called (0, 0) would now be called (a, b) for some other numbers a and b. And yes, we can perform 1D vector transformations on the real number line too. Here's a version of range that sets the origin to 42, not 0: def myrange(start, end=None, step=1): if end is None: start = 42 return range(start, end, step) This is lost on me... Nevertheless, there really is something special about the point 0 on the real number line Agreed. , the point (0,0) on the complex number plane, Also agreed. the point (0,0,0) in the 3D space, (0,0,0,0) in 4D space, etc. It is not just an arbitrary convention that we set the origin to 0. Wrong. The point (0,0,0,...) in some ND space is an arbitrarily chosen position. By this I don't mean to say that the sequence of coordinates consisting of all zeros is arbitrary. The choice of the point *in the real/hypothetical space* that is designated by the sequence of zero coordinates is arbitrary. In other words: to the extent that your arguments that zero-vectors are special are correct, the same applies to zero-points, since vectors are defined as a magnitude and direction *from the origin*. Plain wrong. Vectors are not defined *from any origin*. To put it yet another way: The complex number a+bj is equivalent to the 2D point (a, b) which is equivalent to the 2D vector [a, b]. If (0, 0) shouldn't be considered falsey, neither should [0, 0]. a+bj is not equivalent to the 2D point (a, b). It is possible to define a mapping between complex numbers and a 2D space so that a+bj corresponds to the point (a, b) *under that map*. However there are an infinite number of such possible mappings between the two spaces including a+bj - (a+1, b+1). The significance of zero in real algebra is not that it is the origin but rather that it is the additive and multiplicative zero: a + 0 = a for any real number a a * 0 = 0 for any real number a I'm not sure what you mean by additive and multiplicative zero, you appear to be conflating two different properties here. 0 is the additive *identity*, but 1 is the multiplicative identity: I mean that it has the properties that zero has when used in addition and multiplication: http://en.wikipedia.org/wiki/0_%28number%29#Elementary_algebra a + 0 = a a * 1 = a for any real number a. No. I meant the two properties that I listed. If the RHS must be zero, then there is a unique multiplicative zero, but no unique additive zero: a * 0 = 0
Re: Method default argument whose type is the class not yet defined
On Nov 10, 11:33 am, Jennie namedotpor...@gmail.com wrote: What is the best solution to solve the following problem in Python 3.3? import math class Point: ... def __init__(self, x=0, y=0): ... self.x = x ... self.y = y ... def __sub__(self, other): ... return Point(self.x - other.x, self.y - other.y) ... def distance(self, point=Point()): ... Return the distance from `point`. ... return math.sqrt((self - point).x ** 2 + (self - point).y ** 2) Before you do anything else, introduce a Vector class into your app. The difference between two Points is not a Point; it's a Vector. Create a magnitude() method in your Vector class, then make your Point.distance return the results of Vector.magnitude(self - other). To define the distance of a point from the origin, don't make your distance() method have default arguments; instead, define another method called distance_from_origin(). -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On Nov 11, 4:31 pm, Oscar Benjamin oscar.j.benja...@gmail.com wrote: On 11 November 2012 22:31, Steven D'Aprano Nonsense. The length and direction of a vector is relative to the origin. If the origin is arbitrary, as you claim, then so is the length of the vector. Wrong on all counts. Neither the length not the direction os a vector are relative to any origin. When we choose to express a vector in Cartesian components our representation assumes an orientation for the axes of the coordinate system. Even in this sense, though, the origin itself does not affect the components of the vector. Thank you for pushing back on Steven's imprecise statement that the direction of a vector is relative to the origin. You can't find an angle between two points. That's absurd. You need axes for context. Vectors, points and complex numbers are not equivalent. There are cases in which it is reasonable to think of them as equivalent for a particular purpose. That does not diminish the fundamental differences between them. I looked to wikipedia for clarity, but the definition of a Euclidean vector is somewhat muddy: http://en.wikipedia.org/wiki/Euclidean_vector They say that the formal definition of a vector is a directed line segment. But then they define a free vector as an entity where only the magnitude and direction matter, not the initial point. As you say, it's not unreasonable to treat vectors, points, and complex numbers as equivalent in many circumstances. But, if you're gonna be pedantic, they really are different things. -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On 12/11/2012 00:31, Oscar Benjamin wrote: Plain wrong. Vectors are not defined *from any origin*. So when the Captain says full speed ahead, steer 245 degrees, you haven't the faintest idea where you're going, because you have no origin? -- Cheers. Mark Lawrence. -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On 12 November 2012 01:10, Mark Lawrence breamore...@yahoo.co.uk wrote: On 12/11/2012 00:31, Oscar Benjamin wrote: Plain wrong. Vectors are not defined *from any origin*. So when the Captain says full speed ahead, steer 245 degrees, you haven't the faintest idea where you're going, because you have no origin? As Steve has just explained, the origin has nothing to do with the orientation of the coordinate system. But then I'm assuming you meant that 245 degrees was a bearing relative to North. Was it supposed to be relative to my current angle? Truthfully I wouldn't know what to do without asking the captain a couple more questions. Oscar -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
In article mailman.3570.1352682390.27098.python-l...@python.org, Mark Lawrence breamore...@yahoo.co.uk wrote: On 12/11/2012 00:31, Oscar Benjamin wrote: Plain wrong. Vectors are not defined *from any origin*. So when the Captain says full speed ahead, steer 245 degrees, you haven't the faintest idea where you're going, because you have no origin? Vectors have a length (full speed ahead) and a direction (245 degrees). What they don't have is a fixed location in space. The captain didn't say, Full speed ahead, steer 245 degrees, from 45.0N, 20.0W. In other words, you are correct. The order, full speed ahead, steer 245 degrees, doesn't give you the faintest idea of where you're going. If you were the helmsman, after you executed that order, without any additional information (such as your current location), you would have no idea what piece of land you will hit, or when you will hit it, if you maintain your current course and speed. -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On 12/11/2012 01:18, Oscar Benjamin wrote: On 12 November 2012 01:10, Mark Lawrence breamore...@yahoo.co.uk wrote: On 12/11/2012 00:31, Oscar Benjamin wrote: Plain wrong. Vectors are not defined *from any origin*. So when the Captain says full speed ahead, steer 245 degrees, you haven't the faintest idea where you're going, because you have no origin? As Steve has just explained, the origin has nothing to do with the orientation of the coordinate system. But then I'm assuming you meant that 245 degrees was a bearing relative to North. Was it supposed to be relative to my current angle? Truthfully I wouldn't know what to do without asking the captain a couple more questions. Oscar The only good acedemic is a dead acedemic? -- Cheers. Mark Lawrence. -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On 12/11/2012 01:15, Roy Smith wrote: In article mailman.3570.1352682390.27098.python-l...@python.org, Mark Lawrence breamore...@yahoo.co.uk wrote: On 12/11/2012 00:31, Oscar Benjamin wrote: Plain wrong. Vectors are not defined *from any origin*. So when the Captain says full speed ahead, steer 245 degrees, you haven't the faintest idea where you're going, because you have no origin? Vectors have a length (full speed ahead) and a direction (245 degrees). What they don't have is a fixed location in space. The captain didn't say, Full speed ahead, steer 245 degrees, from 45.0N, 20.0W. In other words, you are correct. The order, full speed ahead, steer 245 degrees, doesn't give you the faintest idea of where you're going. If you were the helmsman, after you executed that order, without any additional information (such as your current location), you would have no idea what piece of land you will hit, or when you will hit it, if you maintain your current course and speed. Thank you for your explanation. -- Cheers. Mark Lawrence. -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
In article mailman.3571.1352683098.27098.python-l...@python.org, Oscar Benjamin oscar.j.benja...@gmail.com wrote: But then I'm assuming you meant that 245 degrees was a bearing relative to North. Was it supposed to be relative to my current angle? Truthfully I wouldn't know what to do without asking the captain a couple more questions. Granted, this requires some domain-specific knowledge, but an order to steer 245 degrees means relative to north (and, technically, it's a heading, not a bearing, but that's another discussion). If the captain wanted you to change you heading relative to your current heading, he would say something like, turn left 10 degrees (that may not be strictly the correct wording). -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On 12 November 2012 01:29, Mark Lawrence breamore...@yahoo.co.uk wrote: On 12/11/2012 01:18, Oscar Benjamin wrote: On 12 November 2012 01:10, Mark Lawrence breamore...@yahoo.co.uk wrote: On 12/11/2012 00:31, Oscar Benjamin wrote: Plain wrong. Vectors are not defined *from any origin*. So when the Captain says full speed ahead, steer 245 degrees, you haven't the faintest idea where you're going, because you have no origin? As Steve has just explained, the origin has nothing to do with the orientation of the coordinate system. But then I'm assuming you meant that 245 degrees was a bearing relative to North. Was it supposed to be relative to my current angle? Truthfully I wouldn't know what to do without asking the captain a couple more questions. The only good acedemic is a dead acedemic? Is that what happens when people ask questions on your ship: it's the plank for him with the questions-askin'! I'm glad you're not my captain. Oscar -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On Mon, 12 Nov 2012 00:31:53 +, Oscar Benjamin wrote: [...] You were right the first time, Chris. A point that happens to coincide with the arbitrarily chosen origin is no more truthy or falsey than any other. A vector of length 0 on the other hand is a very different beast. Nonsense. The length and direction of a vector is relative to the origin. If the origin is arbitrary, as you claim, then so is the length of the vector. Wrong on all counts. Neither the length not the direction os a vector are relative to any origin. When we choose to express a vector in Cartesian components our representation assumes an orientation for the axes of the coordinate system. Even in this sense, though, the origin itself does not affect the components of the vector. Draw a set of axes and mark the vector [1, 1]. Here's a crappy ASCII art diagram, with X marking the head of the vector and a line drawn from the origin to the head. | | X | / | / |/ +-- Now draw a second set of axes with the origin set at the head of that vector. For reference, I leave the previous axis in place. As before, X represents the head of the vector. | | X- | | | | | | +---|-- Note that the body of the vector -- the line from the origin to the head -- is gone. That's because the vector [1, 1] is transformed to the vector [0, 0] under a translation of one unit in both the X and Y directions. The magnitude of the vector under one coordinate system is 1, under the second it is 0. In a nutshell, you can't talk about either *distance* (magnitude) or *direction* without an answer to distance from where? direction relative to what?. I have spent a fair few hours in the past few weeks persuading teenaged Engineering students to maintain a clear distinction between points, vectors and lines. One of the ways that I distinguish vectors from points is to say that a vector is like an arrow but its base has no particular position. A point on the other hand is quite simply a position. Given an origin (an arbitrarily chosen point) we can specify another point using a position vector: a vector from the origin to the point in question. Just because you have spent a lot of time and effort giving people advice doesn't make it *good* advice. [...] Wrong. The point (0,0,0,...) in some ND space is an arbitrarily chosen position. By this I don't mean to say that the sequence of coordinates consisting of all zeros is arbitrary. The choice of the point *in the real/hypothetical space* that is designated by the sequence of zero coordinates is arbitrary. So what? All you are saying is that there is more than one coordinate system, and we can choose the one we like for any problem. Of course we can, and that's a good thing. The significance of zero in real algebra is not that it is the origin but rather that it is the additive and multiplicative zero: a + 0 = a for any real number a a * 0 = 0 for any real number a I'm not sure what you mean by additive and multiplicative zero, you appear to be conflating two different properties here. 0 is the additive *identity*, but 1 is the multiplicative identity: I mean that it has the properties that zero has when used in addition and multiplication: http://en.wikipedia.org/wiki/0_%28number%29#Elementary_algebra I see no reference to additive and multiplicative zero there. Did you make up that terminology? The *identity* element is a common mathematical term, but 1 is the multiplicative identity element. What you are describing is generally known as the absorbing element over multiplication: if X*a = X for any a, then X is an absorbing element under multiplication. http://en.wikipedia.org/wiki/Absorbing_element And by the way, the vector [0, 0] (generalised to however many dimensions you need) is not necessarily the only null (zero) vector. Some vector spaces have many null vectors with non-zero components. http://en.wikipedia.org/wiki/Zero_vector but I digress. The exact terminology doesn't really change anything, since everything you say about vector [0,0] applies equally to the point (0,0) in the Cartesian plane. [...] There is however no meaningful sense in which points (as opposed to vectors) can be added to each other or multiplied by anything, so there is no zero point. I think that the great mathematician Carl Gauss would have something to say about that. Is the text below a quote? No. Points in the plane are equivalent to complex numbers, and you can certainly add and multiply complex numbers. Adding two points is equivalent to a translation; multiplication of a scalar with a point is equivalent to a scale transformation. Multiplying two points is equivalent to complex multiplication, which is a scale + a rotation. The last point is bizarre. Complex multiplication makes no sense when you're trying to think about vectors. Draw a 2D plot and convince yourself that the square of the
Re: Method default argument whose type is the class not yet defined
On Sun, Nov 11, 2012 at 6:33 AM, Jennie namedotpor...@gmail.com wrote: ... def distance(self, point=None): ... p = point if point else Point() I'd go with this one. Definitely not the third one, which mutates the class according to a current global every time a Point is instantiated - could be *extremely* confusing if the name distance were ever rebound. You could also fiddle with the default args: class Point: def __init__(self, x=0, y=0): self.x = x self.y = y def __sub__(self, other): return Point(self.x - other.x, self.y - other.y) def distance(self, point=Point()): return math.sqrt((self - p).x ** 2 + (self - p).y ** 2) Point.distance.__defaults__=Point(), # has to be a tuple ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On 11/10/2012 2:33 PM, Jennie wrote: What is the best solution to solve the following problem in Python 3.3? import math class Point: ... def __init__(self, x=0, y=0): ... self.x = x ... self.y = y ... def __sub__(self, other): ... return Point(self.x - other.x, self.y - other.y) ... def distance(self, point=Point()): ... Return the distance from `point`. ... return math.sqrt((self - point).x ** 2 + (self - point).y ** 2) ... Traceback (most recent call last): File stdin, line 1, in module File stdin, line 5, in Point NameError: name 'Point' is not defined I propose three solutions. The first one: class Point: ... def __init__(self, x=0, y=0): ... self.x = x ... self.y = y ... def __sub__(self, other): ... return Point(self.x - other.x, self.y - other.y) ... def distance(self, point=None): ... p = point if point else Point() ... return math.sqrt((self - p).x ** 2 + (self - p).y ** 2) ... p = Point() p.distance() 0.0 p.distance(Point(3, 4)) 5.0 What I do not like about this one is that it creates a new 0 point each time one is needed. Two solutions: change Point() to point0 in the distance function and create point0 = Point() after the class. -or- instead of p = line, px,py = point.x, point.y if point else 0.0, 0.0 The second one: class Point: ... def __init__(self, x=0, y=0): ... self.x = x ... self.y = y ... def __sub__(self, other): ... return Point(self.x - other.x, self.y - other.y) ... def distance(self, point=Point()): ... return math.sqrt((self - point).x ** 2 + (self - point).y ** 2) ... Point.distance = distance p = Point() p.distance(Point(3, 4)) 5.0 my first thought The last one: class Point: ... def __init__(self, x=0, y=0): ... self.x = x ... self.y = y ... Point.distance = distance ... def __sub__(self, other): ... return Point(self.x - other.x, self.y - other.y) ... def distance(self, point=Point()): ... return math.sqrt((self - point).x ** 2 + (self - point).y ** 2) ... p = Point() p.distance(Point(3, 4)) 5.0 Is there a better solution? -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On 11/10/2012 09:29 PM, Terry Reedy wrote: On 11/10/2012 2:33 PM, Jennie wrote: I propose three solutions. The first one: class Point: ... def __init__(self, x=0, y=0): ... self.x = x ... self.y = y ... def __sub__(self, other): ... return Point(self.x - other.x, self.y - other.y) ... def distance(self, point=None): ... p = point if point else Point() ... return math.sqrt((self - p).x ** 2 + (self - p).y ** 2) What I do not like about this one is that it creates a new 0 point each time one is needed. Two solutions: change Point() to point0 in the distance function and create point0 = Point() after the class. -or- instead of p = line, px,py = point.x, point.y if point else 0.0, 0.0 Thanks, I like the second one :) -- Jennie -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On 11/10/2012 03:51 PM, Jennie wrote: On 11/10/2012 09:29 PM, Terry Reedy wrote: On 11/10/2012 2:33 PM, Jennie wrote: I propose three solutions. The first one: class Point: ... def __init__(self, x=0, y=0): ... self.x = x ... self.y = y ... def __sub__(self, other): ... return Point(self.x - other.x, self.y - other.y) ... def distance(self, point=None): ... p = point if point else Point() ... return math.sqrt((self - p).x ** 2 + (self - p).y ** 2) What I do not like about this one is that it creates a new 0 point each time one is needed. Two solutions: change Point() to point0 in the distance function and create point0 = Point() after the class. -or- instead of p = line, px,py = point.x, point.y if point else 0.0, 0.0 Thanks, I like the second one :) I like the first, once you fix the minor inefficiency in it; add the qualifier is None ... def distance(self, point=None): ... p = point if point is None else Point() ... return math.sqrt((self - p).x ** 2 + (self - p).y ** 2) The advantage it then has over the second one is that the whole class is defined inside the class. -- DaveA -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On Sat, 10 Nov 2012 20:33:05 +0100, Jennie wrote: [...] I propose three solutions. The first one: class Point: ... def __init__(self, x=0, y=0): ... self.x = x ... self.y = y ... def __sub__(self, other): ... return Point(self.x - other.x, self.y - other.y) Don't do this, because it breaks subclassing. Your instance should dynamically get it's own class, not hard-code the name of Point. return self.__class__(self.x - other.x, self.y - other.y) That way, when you subclass Point, you can do arithmetic on the subclass instances and they will do the Right Thing. Note: Python's builtin numeric types don't do this, and it is a damned nuisance: py class MyInt(int): ... pass ... py a, b = MyInt(23), MyInt(42) py assert type(a) is MyInt and type(b) is MyInt py type(a + b) type 'int' Back to your class: ... def distance(self, point=None): ... p = point if point else Point() ... return math.sqrt((self - p).x ** 2 + (self - p).y ** 2) Almost but not quite. I assume that, in a full Point class, you would want Point(0, 0) to count as false in a boolean context. (A falsey value, like None, [], 0.0, etc.) So change the test to an explicit test for None, not just any falsey value: if point is None: point = self.__class__() # Allow subclassing to work. The second one: class Point: ... def __init__(self, x=0, y=0): ... self.x = x ... self.y = y ... def __sub__(self, other): ... return Point(self.x - other.x, self.y - other.y) ... def distance(self, point=Point()): ... return math.sqrt((self - point).x ** 2 + (self - point).y ** 2) ... Point.distance = distance Cute, but ugly and messy. You can inject methods into a class, of course, but that's an awfully big hammer to crack this tiny little nut. Your first solution is better. Here is a variation which, according to your tastes, may count as more or less ugly: inject the default value into the method: class Point: def distance(self, other=None): # None is a placeholder delta = self - other return math.sqrt(delta.x ** 2 + delta.y ** 2) Point.distance.__defaults__ = (Point(),) # In Python 2, use: # Point.distance.__func__.func_defaults = (Point(),) -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On 10 November 2012 19:33, Jennie namedotpor...@gmail.com wrote:
What is the best solution to solve the following problem in Python 3.3?
import math
class Point:
... def __init__(self, x=0, y=0):
... self.x = x
... self.y = y
... def __sub__(self, other):
... return Point(self.x - other.x, self.y - other.y)
... def distance(self, point=Point()):
... Return the distance from `point`.
... return math.sqrt((self - point).x ** 2 + (self - point).y ** 2)
...
Traceback (most recent call last):
File stdin, line 1, in module
File stdin, line 5, in Point
NameError: name 'Point' is not defined
I would use namedtuple and make it so that an ordinary tuple could be
used as in place of a Point instance:
import math
from collections import namedtuple
class Point(namedtuple('Point', ['x', 'y'])):
... def distance(self, other=(0, 0)):
... (myx, myy), (theirx, theiry) = self, other
... return math.sqrt((myx - theirx) ** 2 + (myy - theiry) ** 2)
...
p = Point(3, 4)
p.distance()
5.0
p2 = Point(4, 5)
p.distance(p2)
1.4142135623730951
Oscar
--
http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On Sun, Nov 11, 2012 at 12:13 PM, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: Almost but not quite. I assume that, in a full Point class, you would want Point(0, 0) to count as false in a boolean context. (A falsey value, like None, [], 0.0, etc.) I would not assume that. The origin is a point, just like any other. With a Line class, you could deem a zero-length line to be like a zero-element list, but Point(0,0) is more like the tuple (0,0) which is definitely True. In any case, this would not even matter, beyond unnecessary work; the bug would occur only if you seek the distance to Point(0,0), at which point[1] the code would throw out the incoming Point and go with the default of 0,0. So it'd result in the same distance. ChrisA [1] Sorry, couldn't resist -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On Sat, Nov 10, 2012 at 7:13 PM, Chris Angelico ros...@gmail.com wrote: I would not assume that. The origin is a point, just like any other. With a Line class, you could deem a zero-length line to be like a zero-element list, but Point(0,0) is more like the tuple (0,0) which is definitely True. It's more like the number 0 than the tuple (0,0). 0 is the origin on a 1-dimensional number line. (0,0) is the origin on a 2-dimensional number plane. In fact, it might be pointed out that Point(0, 0) is a generalization of 0+0j, which is equal to 0. -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On Sun, Nov 11, 2012 at 1:43 PM, Ian Kelly ian.g.ke...@gmail.com wrote: On Sat, Nov 10, 2012 at 7:13 PM, Chris Angelico ros...@gmail.com wrote: I would not assume that. The origin is a point, just like any other. With a Line class, you could deem a zero-length line to be like a zero-element list, but Point(0,0) is more like the tuple (0,0) which is definitely True. It's more like the number 0 than the tuple (0,0). 0 is the origin on a 1-dimensional number line. (0,0) is the origin on a 2-dimensional number plane. In fact, it might be pointed out that Point(0, 0) is a generalization of 0+0j, which is equal to 0. Ah, good point. In any case, though, it'd be an utterly inconsequential bug. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
In article mailman.3549.1352601828.27098.python-l...@python.org, Ian Kelly ian.g.ke...@gmail.com wrote: On Sat, Nov 10, 2012 at 7:13 PM, Chris Angelico ros...@gmail.com wrote: I would not assume that. The origin is a point, just like any other. With a Line class, you could deem a zero-length line to be like a zero-element list, but Point(0,0) is more like the tuple (0,0) which is definitely True. It's more like the number 0 than the tuple (0,0). 0 is the origin on a 1-dimensional number line. (0,0) is the origin on a 2-dimensional number plane. In fact, it might be pointed out that Point(0, 0) is a generalization of 0+0j, which is equal to 0. If (0,0) is the origin on a plane, then (0,) should be the origin on a line. If you consider 0 + 0j to be the origin of a plane, then 0 is the origin of a line. Either way is plausible, but you need to be consistent. -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On Sat, Nov 10, 2012 at 7:53 PM, Roy Smith r...@panix.com wrote: In article mailman.3549.1352601828.27098.python-l...@python.org, Ian Kelly ian.g.ke...@gmail.com wrote: On Sat, Nov 10, 2012 at 7:13 PM, Chris Angelico ros...@gmail.com wrote: I would not assume that. The origin is a point, just like any other. With a Line class, you could deem a zero-length line to be like a zero-element list, but Point(0,0) is more like the tuple (0,0) which is definitely True. It's more like the number 0 than the tuple (0,0). 0 is the origin on a 1-dimensional number line. (0,0) is the origin on a 2-dimensional number plane. In fact, it might be pointed out that Point(0, 0) is a generalization of 0+0j, which is equal to 0. If (0,0) is the origin on a plane, then (0,) should be the origin on a line. If you consider 0 + 0j to be the origin of a plane, then 0 is the origin of a line. Either way is plausible, but you need to be consistent. Where I wrote (0,0) is the origin above I was not referring to a point, not a tuple, but I can see how that was confusing. -- http://mail.python.org/mailman/listinfo/python-list
Re: Method default argument whose type is the class not yet defined
On Sat, Nov 10, 2012 at 11:43 PM, Ian Kelly ian.g.ke...@gmail.com wrote: Where I wrote (0,0) is the origin above I was not referring to a point, not a tuple, but I can see how that was confusing. What I meant to say is I *was* referring to a point. Gah! -- http://mail.python.org/mailman/listinfo/python-list
