Re: Tuples vs Lists: Semantic difference (was: Extract String From Enclosing Tuple)
George Sakkis, I agree with the things you say. Sometimes you may have a sequence of uniform data with unknown len (so its index doesn't have semantic meaning). You may want to use it as dict key, so you probably use a tuple meant as just an immutable list. I don't know Ruby, but I think it allows such purposes with a freezing function. Bye, bearophile -- http://mail.python.org/mailman/listinfo/python-list
Re: Extract String From Enclosing Tuple
On 2007-02-28, [EMAIL PROTECTED] [EMAIL PROTECTED] wrote:
import itertools
tuple(itertools.chain((t[0], t2[0].encode('ascii')), t[2:]))
('eco', 'Roads', 0.073969887301348305)
Steven,
As suggested in the previous article, I handled it where the values are
read from the list retrieved from the database. By adding an additional
index of [0] the format is correct.
Thank you all very much,
Rich
--
http://mail.python.org/mailman/listinfo/python-list
Re: Tuples vs Lists: Semantic difference (was: Extract String From Enclosing Tuple)
On Mar 1, 5:02 am, [EMAIL PROTECTED] wrote: I don't know Ruby, but I think it allows such purposes with a freezing function. In ruby all objects can be frozen (freeze is a method on Object, from which all other objects derive), not just Arrays (Arrays == lists in python; ruby has no built-in container equiv. to tuple). But that's more of an implementation detail rather than anthing to do with the structure/semantics of a certain type of object (e.g., a String can be frozen, a Hash can be frozen, c). Regards, Jordan -- http://mail.python.org/mailman/listinfo/python-list
Extract String From Enclosing Tuple
I'm a bit embarrassed to have to ask for help on this, but I'm not finding
the solution in the docs I have here.
Data are assembled for writing to a database table. A representative tuple
looks like this:
('eco', (u'Roads',), 0.073969887301348305)
Pysqlite doesn't like the format of the middle term:
pysqlite2.dbapi2.InterfaceError: Error binding parameter 1 - probably
unsupported type.
I want to extract the 'Roads', part from the double-quoted enclosing
tuple. The unicode part should be automatically removed when the string is
printed, but I suspect it's the double quotes and extra parentheses that are
the problem. I know that tuples are immutable, but I thought that I could
slice it. If so, I'm not doing it correctly, because each attempt results in
TypeError: unsubscriptable object
Even when I assign that middle term to a variable before assembling the
tuple for insertion in the database, I just cannot get the job done. Whether
in the tuple of three terms or by itself, I haven't applied the proper
technique.
Insight appreciated.
Rich
--
http://mail.python.org/mailman/listinfo/python-list
Re: Extract String From Enclosing Tuple
[EMAIL PROTECTED] a écrit :
I'm a bit embarrassed to have to ask for help on this, but I'm not finding
the solution in the docs I have here.
Data are assembled for writing to a database table. A representative tuple
looks like this:
('eco', (u'Roads',), 0.073969887301348305)
Pysqlite doesn't like the format of the middle term:
pysqlite2.dbapi2.InterfaceError: Error binding parameter 1 - probably
unsupported type.
I want to extract the 'Roads', part from the double-quoted enclosing
tuple. The unicode part should be automatically removed when the string is
printed, but I suspect it's the double quotes and extra parentheses that are
the problem. I know that tuples are immutable, but I thought that I could
slice it. If so, I'm not doing it correctly, because each attempt results in
TypeError: unsubscriptable object
Where do you get your data from ? MHO is that you'd better handle the
problem at the source (ie : dont put a string representation of a tuple
in your data) instead of trying to fix it afterward.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Extract String From Enclosing Tuple
On Feb 28, 12:40 pm, [EMAIL PROTECTED] wrote:
I'm a bit embarrassed to have to ask for help on this, but I'm not finding
the solution in the docs I have here.
Data are assembled for writing to a database table. A representative tuple
looks like this:
('eco', (u'Roads',), 0.073969887301348305)
Pysqlite doesn't like the format of the middle term:
pysqlite2.dbapi2.InterfaceError: Error binding parameter 1 - probably
unsupported type.
I want to extract the 'Roads', part from the double-quoted enclosing
tuple.
(snipped)
Perhaps something like:
t = ('eco', (u'Roads',), 0.073969887301348305)
t2 = eval(t[1], { '__builtins__' : {} }, {} )
t2
(u'Roads',)
t2[0].encode('ascii')
'Roads'
import itertools
tuple(itertools.chain((t[0], t2[0].encode('ascii')), t[2:]))
('eco', 'Roads', 0.073969887301348305)
tuple(itertools.chain((t[0], (t2[0].encode('ascii'),)), t[2:]))
('eco', ('Roads',), 0.073969887301348305)
--
Hope this helps,
Steven
--
http://mail.python.org/mailman/listinfo/python-list
Re: Extract String From Enclosing Tuple
[EMAIL PROTECTED] writes:
Data are assembled for writing to a database table. A
representative tuple looks like this:
('eco', (u'Roads',), 0.073969887301348305)
You refer to the second item as a tuple later, but it's not; it's
now just a string (not even a unicode string). Whatever has assembled
these has effectively lost the structure of the data and you are now
left with three items in a tuple: string, string, float.
Some RDBMSs can store a structure of multiple values in one value;
SQLite cannot. The usual solution for this limitation is to take these
structural values and store the component values as separate rows of a
different table, and have each of those rows refer back to the
identifying key of the original table so they can be joined easily.
E.g., I might conceptually think of order records as the following
tuples, with further tuples-of-tuples for the items on each order:
orders = (
# fields: id, cust_code, date, order_items
(1, cust1234, 2007-02-15, ((item002, 1), (item005, 3), (item007, 1))),
(2, cust4567, 2007-02-19, ((item001, 5), (item005, 2))),
)
Since I can't store those as-is in SQLite, I would need to restructure
the tables: separate the order items to separate rows in a dedicated
table, and refer to the order table key in each order item row.
orders = (
# fields: id, cust_code, date
(1, cust1234, 2007-02-15),
(2, cust4567, 2007-02-19),
)
order_items = (
# fields: id, order_id, item_code, quantity
(1, 1, item002, 1),
(2, 1, item005, 3),
(3, 1, item007, 1),
(4, 2, item001, 5),
(5, 2, item005, 2),
)
Then you can use SQL JOIN clauses as necessary, with the
order_item.order_id field a foreign key into the order table.
--
\ I moved into an all-electric house. I forgot and left the |
`\ porch light on all day. When I got home the front door wouldn't |
_o__) open. -- Steven Wright |
Ben Finney
--
http://mail.python.org/mailman/listinfo/python-list
Re: Extract String From Enclosing Tuple
Ben Finney [EMAIL PROTECTED] writes: orders = ( [...] ) order_items = ( [...] ) For clarity, these sequences of records should be lists (with each item being a tuple containing the record fields), not tuples. A tuple implies a meaning associated with each position in the sequence (like a record with a positional meaning for each field), a list implies the opposite (a sequence with order but not meaning associated with each position). -- \ If you were going to shoot a mime, would you use a silencer? | `\ -- Steven Wright | _o__) | Ben Finney -- http://mail.python.org/mailman/listinfo/python-list
Re: Extract String From Enclosing Tuple
Ben Finney wrote: A tuple implies a meaning associated with each position in the sequence (like a record with a positional meaning for each field), a list implies the opposite (a sequence with order but not meaning associated with each position). Explain. I know tuples as immutable lists ... BTW, don't confuse python tuples with tuples from mathematics. Regards, Björn -- BOFH excuse #214: Fluorescent lights are generating negative ions. If turning them off doesn't work, take them out and put tin foil on the ends. -- http://mail.python.org/mailman/listinfo/python-list
Tuples vs Lists: Semantic difference (was: Extract String From Enclosing Tuple)
Bjoern Schliessmann [EMAIL PROTECTED] writes: Ben Finney wrote: A tuple implies a meaning associated with each position in the sequence (like a record with a positional meaning for each field), a list implies the opposite (a sequence with order but not meaning associated with each position). Explain. Well, since you ask so politely :-) I know tuples as immutable lists ... That's a common misconception. Tuples are intended for use as heterogeneous data structures: every index in the sequence *means* something, a semantic meaning applied to the item at that index. It's for this reason that a tuple is immutable: removing items, inserting them in the middle, etc. would imply that the index doesn't have semantic meaning for the structure, which is not true. Lists are intended for use as homogeneous sequences: not that every value is of the same type, but that a particular index in the sequence doesn't *mean* anything about the semantic interpretation of the item at that position. It's for this reason that a list is mutable: since the index of an item has no semantic meaning, inserting new items or removing them from anywhere in the sequence doesn't alter the meaning of the structure. James Tauber explains further: URL:http://jtauber.com/blog/2006/04/15/python_tuples_are_not_just_constant_lists -- \ You can be a victor without having victims. -- Harriet Woods | `\ | _o__) | Ben Finney -- http://mail.python.org/mailman/listinfo/python-list
Re: Tuples vs Lists: Semantic difference (was: Extract String From Enclosing Tuple)
On Feb 28, 10:45 pm, Ben Finney [EMAIL PROTECTED] wrote: Bjoern Schliessmann [EMAIL PROTECTED] writes: I know tuples as immutable lists ... That's a common misconception. And this catch phrase, that's a common misconception, is a common aping of the BDFL's take on this. As several long threads have shown, it is a highly controversial topic and claiming that one side has misconceived it doesn't make it more true than a Catholic claiming that Protestants are misconceived about the True Christianity or vice versa. Tuples are intended for use as heterogeneous data structures: every index in the sequence *means* something, a semantic meaning applied to the item at that index. It's for this reason that a tuple is immutable: removing items, inserting them in the middle, etc. would imply that the index doesn't have semantic meaning for the structure, which is not true. Lists are intended for use as homogeneous sequences: not that every value is of the same type, but that a particular index in the sequence doesn't *mean* anything about the semantic interpretation of the item at that position. It's for this reason that a list is mutable: since the index of an item has no semantic meaning, inserting new items or removing them from anywhere in the sequence doesn't alter the meaning of the structure. James Tauber explains further: URL:http://jtauber.com/blog/2006/04/15/python_tuples_are_not_just_constan... Nice, that's a good summary of the straw man arguments about the true distinction between tuples and lists. Now can you please explain why an heterogeneous data structure: 1) does not support __setitem__, changing the value of an existing item from 3 to 4, 2) supports iteration over its (heterogeneneous) elements, but not an index() method, and 3) why using indices rather than names for implied semantics is a good idea anyway. As for addition/removal/insertion of elements not making sense for a heterogeneous data structure, have you heard of database schema change ? Heterogeneous data structures are well known for several decades now; they are commonly spelled records though, not tuples, and have a more reasonable API to support their semantics. George -- http://mail.python.org/mailman/listinfo/python-list
