Re: quick beginners List comprehension question
You can try this import random class foo: def __init__(self): self.bar = random.randint(1,100) def getbar(ls,i): ls.append(foo()) ls[i].bar = ls[i].bar * 3 ls = [] [getbar(ls,i) for i in range(10)] On Thu, Jan 22, 2009 at 4:45 AM, Diez B. Roggisch wrote: > MRAB schrieb: > >> Diez B. Roggisch wrote: >> >>> Dr Mephesto wrote: >>> >>> Hi, Im new to python, and OOP, and am trying to get a handle on list comprehension. Say I have a class Foo with a property called bar: class Foo: def __init__(self): self.bar = random.randint(1,100) and then I make a list of these objects: Newlist = [] for x in range(10): Newlist.append(Foo()) Now, suppose I wanted to triple the value of 'bar', I could always do: for x in range(10): Newlist[x].bar = Newlist[x].bar * 3 but can I do this using list comprehension? Thanks in Advance! >>> >>> No, as such, because list-comprehensions require you to have an >>> *expression* >>> in front of the iteration: >>> >>> resultlist = [ for in ] >>> >>> Now what you of course can do is this: >>> >>> def multiply(item): >>>item.bar = item.bar * 3 >>> >>> [multiply(i) for i in items] >>> >>> However, doing this will make python produce a list of None-references - >>> which is a waste. It's up to you if you care about that, but generally it >>> is frowned upon because of that, and the fact that the conciseness of the >>> list-comp here isn't really helping with the readability. >>> >>> If you had: >> >> def multiply(item): >>item.bar = item.bar * 3 >>return item >> >> then: >> >> [multiply(i) for i in items] >> >> would return items. Still a bad idea, though, because you're using a list >> comprehension for its side-effect. >> > > And redundant, which was the reason I ommited it. > > > Diez > -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
Re: quick beginners List comprehension question
MRAB schrieb: Diez B. Roggisch wrote: Dr Mephesto wrote: Hi, Im new to python, and OOP, and am trying to get a handle on list comprehension. Say I have a class Foo with a property called bar: class Foo: def __init__(self): self.bar = random.randint(1,100) and then I make a list of these objects: Newlist = [] for x in range(10): Newlist.append(Foo()) Now, suppose I wanted to triple the value of 'bar', I could always do: for x in range(10): Newlist[x].bar = Newlist[x].bar * 3 but can I do this using list comprehension? Thanks in Advance! No, as such, because list-comprehensions require you to have an *expression* in front of the iteration: resultlist = [ for in ] Now what you of course can do is this: def multiply(item): item.bar = item.bar * 3 [multiply(i) for i in items] However, doing this will make python produce a list of None-references - which is a waste. It's up to you if you care about that, but generally it is frowned upon because of that, and the fact that the conciseness of the list-comp here isn't really helping with the readability. If you had: def multiply(item): item.bar = item.bar * 3 return item then: [multiply(i) for i in items] would return items. Still a bad idea, though, because you're using a list comprehension for its side-effect. And redundant, which was the reason I ommited it. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: quick beginners List comprehension question
Dr Mephesto wrote: Hi, Im new to python, and OOP, and am trying to get a handle on list comprehension. Say I have a class Foo with a property called bar: class Foo: def __init__(self): self.bar = random.randint(1,100) and then I make a list of these objects: Newlist = [] for x in range(10): Newlist.append(Foo()) Constructing this list is the appropriate place for a comprehension. Newlist = [Foo() for _ in range(10)] Now, suppose I wanted to triple the value of 'bar', I could always do: for x in range(10): Newlist[x].bar = Newlist[x].bar * 3 Use MRAB's replacement for this. but can I do this using list comprehension? Don't, for reasons given by others. tjr -- http://mail.python.org/mailman/listinfo/python-list
Re: quick beginners List comprehension question
On Jan 21, 2009, at 11:52 AM, Lou Pecora wrote: In article , Philip Semanchuk wrote: Other answers have been good; to them I'll add the comment that list comprehensions are for *constructing* lists, not manipulating the elements thereof. HTH Philip Well this seems to work just fine. What am I missing: A=[1,2,3] print A A=[2*a for a in A] print A You haven't manipulated the list A, you've simply overwritten it with a new list. -- http://mail.python.org/mailman/listinfo/python-list
Re: quick beginners List comprehension question
Lou Pecora wrote: > In article , > Philip Semanchuk wrote: > >> Other answers have been good; to them I'll add the comment that list >> comprehensions are for *constructing* lists, not manipulating the >> elements thereof. >> >> HTH >> Philip > > > Well this seems to work just fine. What am I missing: > >A=[1,2,3] >print A >A=[2*a for a in A] >print A > The fact that the lists to be multiplied are attributes of a list of objects, and therefore aren't themselves a list. Look more closely at the original poster's question. regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: quick beginners List comprehension question
In article , Philip Semanchuk wrote: > > Other answers have been good; to them I'll add the comment that list > comprehensions are for *constructing* lists, not manipulating the > elements thereof. > > HTH > Philip Well this seems to work just fine. What am I missing: A=[1,2,3] print A A=[2*a for a in A] print A -- -- Lou Pecora -- http://mail.python.org/mailman/listinfo/python-list
Re: quick beginners List comprehension question
Diez B. Roggisch wrote: Dr Mephesto wrote: Hi, Im new to python, and OOP, and am trying to get a handle on list comprehension. Say I have a class Foo with a property called bar: class Foo: def __init__(self): self.bar = random.randint(1,100) and then I make a list of these objects: Newlist = [] for x in range(10): Newlist.append(Foo()) Now, suppose I wanted to triple the value of 'bar', I could always do: for x in range(10): Newlist[x].bar = Newlist[x].bar * 3 but can I do this using list comprehension? Thanks in Advance! No, as such, because list-comprehensions require you to have an *expression* in front of the iteration: resultlist = [ for in ] Now what you of course can do is this: def multiply(item): item.bar = item.bar * 3 [multiply(i) for i in items] However, doing this will make python produce a list of None-references - which is a waste. It's up to you if you care about that, but generally it is frowned upon because of that, and the fact that the conciseness of the list-comp here isn't really helping with the readability. If you had: def multiply(item): item.bar = item.bar * 3 return item then: [multiply(i) for i in items] would return items. Still a bad idea, though, because you're using a list comprehension for its side-effect. -- http://mail.python.org/mailman/listinfo/python-list
Re: quick beginners List comprehension question
On Jan 21, 2009, at 10:52 AM, Dr Mephesto wrote: Hi, Im new to python, and OOP, and am trying to get a handle on list comprehension. Say I have a class Foo with a property called bar: class Foo: def __init__(self): self.bar = random.randint(1,100) and then I make a list of these objects: Newlist = [] for x in range(10): Newlist.append(Foo()) Now, suppose I wanted to triple the value of 'bar', I could always do: for x in range(10): Newlist[x].bar = Newlist[x].bar * 3 but can I do this using list comprehension? Thanks in Advance! Other answers have been good; to them I'll add the comment that list comprehensions are for *constructing* lists, not manipulating the elements thereof. HTH Philip -- http://mail.python.org/mailman/listinfo/python-list
Re: quick beginners List comprehension question
Dr Mephesto wrote: Hi, Im new to python, and OOP, and am trying to get a handle on list comprehension. Say I have a class Foo with a property called bar: class Foo: def __init__(self): self.bar = random.randint(1,100) and then I make a list of these objects: Newlist = [] for x in range(10): Newlist.append(Foo()) Now, suppose I wanted to triple the value of 'bar', I could always do: for x in range(10): Newlist[x].bar = Newlist[x].bar * 3 but can I do this using list comprehension? Thanks in Advance! You could reduce that to: for x in Newlist: x.bar *= 3 but I don't think you could do it with list comprehension. -- http://mail.python.org/mailman/listinfo/python-list
Re: quick beginners List comprehension question
Dr Mephesto wrote: > Hi, > Im new to python, and OOP, and am trying to get a handle on list > comprehension. > > Say I have a class Foo with a property called bar: > > class Foo: > def __init__(self): > self.bar = random.randint(1,100) > > and then I make a list of these objects: > > Newlist = [] > for x in range(10): > Newlist.append(Foo()) > > Now, suppose I wanted to triple the value of 'bar', I could always do: > > for x in range(10): > Newlist[x].bar = Newlist[x].bar * 3 > > but can I do this using list comprehension? Thanks in Advance! No, as such, because list-comprehensions require you to have an *expression* in front of the iteration: resultlist = [ for in ] Now what you of course can do is this: def multiply(item): item.bar = item.bar * 3 [multiply(i) for i in items] However, doing this will make python produce a list of None-references - which is a waste. It's up to you if you care about that, but generally it is frowned upon because of that, and the fact that the conciseness of the list-comp here isn't really helping with the readability. Diez -- http://mail.python.org/mailman/listinfo/python-list
quick beginners List comprehension question
Hi, Im new to python, and OOP, and am trying to get a handle on list comprehension. Say I have a class Foo with a property called bar: class Foo: def __init__(self): self.bar = random.randint(1,100) and then I make a list of these objects: Newlist = [] for x in range(10): Newlist.append(Foo()) Now, suppose I wanted to triple the value of 'bar', I could always do: for x in range(10): Newlist[x].bar = Newlist[x].bar * 3 but can I do this using list comprehension? Thanks in Advance! -- http://mail.python.org/mailman/listinfo/python-list
