in [Python]

From: Steve Howell on 2 Apr 2010 00:16 On Mar 30, 8:40 am, gentlestone <tibor.b... (a)hotmail.com> wrote:> Hi, how can I write the popular C/JAVA syntax in Python? > > Java example: > return (a==b) ? 'Yes' : 'No' > > ; first idea is: > return ('No','Yes')[bool(a==b)] > > Is there a more elegant/common python expression for this? The ironic thing about the ternary operator is that it is not really ternary; it's binary. Even just making an expression from a binary operator inevitably leads to syntax hell. There is a principle of programming that I would like to coin, which is the "Tyranny of Three." It is impossible to code for any expression that has three possible values in any kind of elegant way. It's just impossible. Try to code the bowling game without tearing out your teeth--three conditions: strike, spare, or normal. The tyranny of three is that 3 is too small for an elegant N-based solution and too large for a simple condition.
From: Steven D'Aprano on 2 Apr 2010 05:04 On Thu, 01 Apr 2010 21:16:18 -0700, Steve Howell wrote: > The ironic thing about the ternary operator is that it is not really > ternary; it's binary. Even just making an expression from a binary > operator inevitably leads to syntax hell. > > There is a principle of programming that I would like to coin, which is > the "Tyranny of Three." > > It is impossible to code for any expression that has three possible > values in any kind of elegant way. It's just impossible. Try to code > the bowling game without tearing out your teeth--three conditions: > strike, spare, or normal. > > The tyranny of three is that 3 is too small for an elegant N-based > solution and too large for a simple condition. I'm afraid I don't understand any of that. Can you explain further? How is the ternary operator "not really ternary, it's binary"? It requires three arguments, not two, which makes it ternary. In Python syntax: value1 if flag else value2 or in C: flag ? value1 : value2 You say that "Even just making an expression from a binary operator inevitably leads to syntax hell." I don't understand what you mean. What is so hellish about any of these? a + b # infix a b + # postfix, familiar to anyone who has programmed HP calculators add(a, b) # prefix function notation + a b # prefix add a to b # verbose English-like Well, perhaps the infix notation counts as "unusual", and the last as "too verbose", but hellish? -- Steven
From: Duncan Booth on 2 Apr 2010 07:20 Steven D'Aprano <steve (a)REMOVE-THIS-cybersource.com.au> wrote:> Yes, I agree, we should be using the previously well known syntax: > > condition -> value_if_true, value_if_false > > which was introduced by BCPL in 1966. > What, not this? VALOF TEST condition THEN RESULTIS value_if_true ELSE RESULTIS value_if_false which was also introduced by BCPL in 1966. :^)
From: Steve Howell on 2 Apr 2010 10:05 On Apr 2, 2:04 am, Steven D'Aprano <st... (a)REMOVE-THIS-cybersource.com.au> wrote: > On Thu, 01 Apr 2010 21:16:18 -0700, Steve Howell wrote: > > The ironic thing about the ternary operator is that it is not really > > ternary; it's binary. Even just making an expression from a binary > > operator inevitably leads to syntax hell. > > > There is a principle of programming that I would like to coin, which is > > the "Tyranny of Three." > > > It is impossible to code for any expression that has three possible > > values in any kind of elegant way. It's just impossible. Try to code > > the bowling game without tearing out your teeth--three conditions: > > strike, spare, or normal. > > > The tyranny of three is that 3 is too small for an elegant N-based > > solution and too large for a simple condition. > > I'm afraid I don't understand any of that. Can you explain further? > > How is the ternary operator "not really ternary, it's binary"? It > requires three arguments, not two, which makes it ternary. In Python > syntax: > Of course, I understand that the ternary operator has three arguments, but it only has two possible outcomes. You asked me to elaborate on the "Tyranny of Three." Let's say you have three possible outcomes. In some languages you would write something like this: mark = (rolls == 1) && (pins == 10) ? 'strike' : (rolls == 2) && (pins == 10) ? 'spare' : 'normal' Many people consider the above very ugly, so they write it like so: if pins == 10: if rolls == 1: return 'strike' else: return 'spare' else: return 'normal' Then the next programmer comes along and "cleans up": if pins == 10: return 'strike' if rolls == 1 else 'spare' else: return 'normal' Then there is this alternative: if rolls == 2: return 'spare' if pins == 10 else 'normal' else: return 'strike' And then: if rolls == 2: if pins == 10 return 'spare' else return 'normal else: return 'strike' Or even this: return 'strike' if rolls == 1 else ('spare' if pins == 10 else 'normal') The "Tyranny of Three" refers to a problem where there are an infinite number of valid solutions, but none of them have any essential beauty, so they lead to endless nitpicking and code churn.
From: Steve Holden on 2 Apr 2010 10:21
Steve Howell wrote: > On Apr 2, 2:04 am, Steven D'Aprano <st... (a)REMOVE-THIS-> cybersource.com.au> wrote: [...] >> How is the ternary operator "not really ternary, it's binary"? It >> requires three arguments, not two, which makes it ternary. In Python >> syntax: >> > > Of course, I understand that the ternary operator has three arguments, > but it only has two possible outcomes. > That doesn't make it a binary operator. Otherwise what's long multiplication, which has an infinite number of possible outcomes? > You asked me to elaborate on the "Tyranny of Three." Let's say you > have three possible outcomes. > > In some languages you would write something like this: > > mark = (rolls == 1) && (pins == 10) ? 'strike' : > (rolls == 2) && (pins == 10) ? 'spare' : > 'normal' > > Many people consider the above very ugly, so they write it like so: > > if pins == 10: > if rolls == 1: > return 'strike' > else: > return 'spare' > else: > return 'normal' > > Then the next programmer comes along and "cleans up": > > if pins == 10: > return 'strike' if rolls == 1 else 'spare' > else: > return 'normal' > > Then there is this alternative: > > if rolls == 2: > return 'spare' if pins == 10 else 'normal' > else: > return 'strike' > > And then: > > if rolls == 2: > if pins == 10 > return 'spare' > else > return 'normal > else: > return 'strike' > > Or even this: > > return 'strike' if rolls == 1 else ('spare' if pins == 10 else > 'normal') > > The "Tyranny of Three" refers to a problem where there are an infinite > number of valid solutions, but none of them have any essential beauty, > so they lead to endless nitpicking and code churn. > The Real Programmer (tm) simply chooses one, implements it and moves on. While philosophical discussions are interesting they don't pay the bills. regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 See PyCon Talks from Atlanta 2010 http://pycon.blip.tv/ Holden Web LLC http://www.holdenweb.com/ UPCOMING EVENTS: http://holdenweb.eventbrite.com/ |