FAQ
I have the argument items in my class room.

class room:
def __init__(self, name, description, items*):

I thought I remembered from a tutorial I read once, and I've read so
many I feel like an expert of them, that putting a little star* above
an item makes it accept the argument as a list. But when I tried this,
I got an invalid syntax error message.

So:
Question 1: How can I create an argument that accepts a list of
variable?

Question 2: How do I signify to accept each item as one list? (my bet's
on using another set of parens)

Question 3: Or am I going about this all wrong and should always create
a list before the fuction call and use the list in the function call?

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  • ArdPy at Oct 30, 2006 at 3:10 am

    Hakusa at gmail.com wrote:
    I have the argument items in my class room.

    class room:
    def __init__(self, name, description, items*):

    I thought I remembered from a tutorial I read once, and I've read so
    many I feel like an expert of them, that putting a little star* above
    an item makes it accept the argument as a list. But when I tried this,
    I got an invalid syntax error message.
    There is an error in the syntax the star must prefix the variable name
    not suffix it.
    Then the items variable will accept the parameter value as a tuple.
    So:
    Question 1: How can I create an argument that accepts a list of
    variable?
    It is not clear whether you want to accept a list variable or an
    arbitrary number of values as parameter
    to the function. So I will explain each case.
    def ex(name,*items):
    Here name, description and items can all accept lists as arguments.
    Additionally items can accept arbitrary
    number of arguments even of different types.
    Ex:
    ex([10,12],12,13.3,'Python')
    Question 2: How do I signify to accept each item as one list? (my bet's
    on using another set of parens)
    I have answered this already.
    Question 3: Or am I going about this all wrong and should always create
    a list before the fuction call and use the list in the function call?
    No need u can pass a list directly into the call.
  • Hakusa at Oct 30, 2006 at 4:18 am

    ArdPy wrote:
    There is an error in the syntax the star must prefix the variable name
    not suffix it.
    Then the items variable will accept the parameter value as a tuple.
    Hmm, tuples are immutable, right? I need something mutable so that when
    the player picks up an item, it's no longer in the room.

    Besides which, I've been playing with this whole thing in the
    interactive interpreter:
    def shuf(x,*foo, **bar):
    ... return x, foo
    ...
    x,foo,bar = shuf(1,2,3,4,5,6,7,8)
    Traceback (most recent call last):
    File "<interactive input>", line 1, in <module>
    ValueError: need more than 2 values to unpack

    I remember that two stars is for the second tuple, but how do I
    distinguish between what var is for which tuple? I've tried using
    brackets, braces, prenthisies, nothing seems to work. So what I am
    doing wrong?
  • ArdPy at Oct 30, 2006 at 6:21 am

    Hakusa at gmail.com wrote:
    ArdPy wrote:
    There is an error in the syntax the star must prefix the variable name
    not suffix it.
    Then the items variable will accept the parameter value as a tuple.
    Hmm, tuples are immutable, right? I need something mutable so that when
    the player picks up an item, it's no longer in the room.

    Besides which, I've been playing with this whole thing in the
    interactive interpreter:
    def shuf(x,*foo, **bar):
    ... return x, foo
    ...
    x,foo,bar = shuf(1,2,3,4,5,6,7,8)
    Traceback (most recent call last):
    File "<interactive input>", line 1, in <module>
    ValueError: need more than 2 values to unpack

    I remember that two stars is for the second tuple, but how do I
    distinguish between what var is for which tuple? I've tried using
    brackets, braces, prenthisies, nothing seems to work. So what I am
    doing wrong?
    The thing is that the parameter 'bar' is actually a dictionary object
    so the right way of calling shuf is then
    x,foo,bar = shuf(1,2,3,4,5,a=6,b=7,c=8)
  • Hakusa at Oct 30, 2006 at 6:29 am
    Thank you very much for your information.
  • Fredrik Lundh at Oct 30, 2006 at 6:59 am

    Hakusa at gmail.com wrote:

    ArdPy wrote:
    There is an error in the syntax the star must prefix the variable name
    not suffix it.
    Then the items variable will accept the parameter value as a tuple.
    Hmm, tuples are immutable, right? I need something mutable so that when
    the player picks up an item, it's no longer in the room.
    the *args syntax is used to capture extra positional arguments, so you
    can deal with them later. consider the following code:

    def func(*args):
    args.append(4)

    func(1, 2, 3)

    where do you expect the "4" to go?

    if you want to *store* the items in an *instance* variable, you can
    trivially convert it to a list by passing it to the list() function:

    class Room:
    def __init__(self, name, *items):
    self.name = name
    self.items = list(items)

    r = Room("hall", "old shoe", "kitten", "vegetables")
    r.items.append("a wallclock round in metal")
    Besides which, I've been playing with this whole thing in the
    interactive interpreter:
    def shuf(x,*foo, **bar):
    ... return x, foo
    ...
    x,foo,bar = shuf(1,2,3,4,5,6,7,8)
    Traceback (most recent call last):
    File "<interactive input>", line 1, in <module>
    ValueError: need more than 2 values to unpack
    if you compare the return statement inside the function with the
    assignment, do you see any notable differences?
    I remember that two stars is for the second tuple
    if you're an expert on tutorials, surely you should be able to find one
    and look this up in no time at all?

    (the **bar syntax captures extra *keyword* arguments)
    So what I am doing wrong?
    programming by trial and error?

    </F>
  • Hieu Hoang at Oct 30, 2006 at 5:15 am
    Hi,

    The shuf() function returns two values: x and foo.
    The command
    x,foo,bar=shuf(1,2,3,4,5,6,7,8)
    becomes
    x,foo,bar=x,foo
    So, this command assigns 2 values (x and foo) to 3 variables (x, foo
    and bar), which raises that exception. I'm not sure why python says
    "need more than 2
    values to unpack" not "need 3 values" though.

    Hope this help,
    Rooy
  • Fredrik Lundh at Oct 30, 2006 at 10:58 am

    Hieu Hoang wrote:

    So, this command assigns 2 values (x and foo) to 3 variables (x, foo
    and bar), which raises that exception. I'm not sure why python says
    "need more than 2 values to unpack" not "need 3 values" though.
    probably because if you look at the call site

    x,foo,bar=shuf(1,2,3,4,5,6,7,8)

    it's more helpful to learn that "shuf" returned 2 values than to learn
    that the code you're looking at requires three values.

    </F>

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postedOct 30, '06 at 1:59a
activeOct 30, '06 at 10:58a
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