Help with Dictionaries and Classes requested please.

**Neil Cerutti** · Aug 9 '07, 12:45 PM

Re: Help with Dictionaries and Classes requested please.

On 2007-08-09, special_dragonf ly <Dominic@PLEASE ASK.co.ukwrote:

Is there anyway for python to consider the values within a
string when entering the data into a dictionary. I know that
isn't very clear so here's an example:
>
class MyClass(object) :
def __init__(self,n ame="",age=""):
self.name=name
self.age=age
>
data="Gary,50"
d={0:[MyClass(data)]}
data="Adam,25"
d[0].append(MyClass (data))
>
The data is coming from a text file working on a line by line
basis. I've just tried and I'm just getting the full string in
the first field. That seems logical, now I don't want it to
though!

That's what happens if you use 0 for the key every time. ;)

If you're trying to map between ages and lists of names, then
you'll want a little helper function to manage the lists for you.

multidict_add(m dict, key, value):
if key in mdict:
mdict[key].append(value)
else:
mdict[key] = [value]

d = {}
multidict_add(d , 50, "Gary")
multidict_add(d , 50, "Guido")
multidict_add(d , 25, "Adam")

Now you'll get a list of names for every age.

>>d[50]

["Gary", "Guido"]

You can use the same function to build a mapping from names to
lists of ages.

d = {}
multidict_add(d , "Gary", 50)
multidict_add(d , "Gary", 23)
multidict_add(d , "Adam", 25)

>>d["Gary"]

[50, 23]

--
Neil Cerutti
The choir invites any member of the congregation who enjoys sinning to join
the choir. --Church Bulletin Blooper

**Ben Finney** · Aug 9 '07, 01:35 PM

Re: Help with Dictionaries and Classes requested please.

"special_dragon fly" <Dominic@PLEASE ASK.co.ukwrites :

I've managed to solve the problem, I really was just being a
dunce.

Doubtful; but at this stage we can't tell, because we still don't know
what it is you're actually trying to *do*.

Here's how incase anyone is wondering:
>
class MyClass:
def __init__(self):
name=""
dict={}
dict[0]=[]
dict[0].append(MyClass ())
dict[0][0].name="Hello"
print dict[0][0].name

It's not clear why you are using the value 0 for a dictionary key
here; nor why you're assigning an attribute to an object after
creating the object. Neither of them are errors, but without context
it's hard to know what advice to give.

--
\ "When we call others dogmatic, what we really object to is |
`\ their holding dogmas that are different from our own." -- |
_o__) Charles Issawi |
Ben Finney

**Bruno Desthuilliers** · Aug 9 '07, 01:45 PM

Re: Help with Dictionaries and Classes requested please.

Neil Cerutti a écrit :

On 2007-08-09, special_dragonf ly <Dominic@PLEASE ASK.co.ukwrote:

>Is there anyway for python to consider the values within a
>string when entering the data into a dictionary. I know that
>isn't very clear so here's an example:
>>
>class MyClass(object) :
> def __init__(self,n ame="",age=""):
> self.name=name
> self.age=age
>>
>data="Gary,5 0"
>d={0:[MyClass(data)]}
>data="Adam,2 5"
>d[0].append(MyClass (data))
>>
>The data is coming from a text file working on a line by line
>basis. I've just tried and I'm just getting the full string in
>the first field. That seems logical, now I don't want it to
>though!

>
That's what happens if you use 0 for the key every time. ;)

Hmmm... Neil, I may be wrong but I think you didn't get the point here.
As I understand it, Dominic's problem is that it gets strings like
"Gary,50" and would like to call MyClass initializer this way :
MyClass("Gary", "50")

**Neil Cerutti** · Aug 9 '07, 01:55 PM

Re: Help with Dictionaries and Classes requested please.

On 2007-08-09, Bruno Desthuilliers <bruno.42.desth uilliers@wtf.we bsiteburo.oops. comwrote:

Neil Cerutti a écrit :

>On 2007-08-09, special_dragonf ly <Dominic@PLEASE ASK.co.ukwrote:

>>Is there anyway for python to consider the values within a
>>string when entering the data into a dictionary. I know that
>>isn't very clear so here's an example:
>>>
>>class MyClass(object) :
>> def __init__(self,n ame="",age=""):
>> self.name=name
>> self.age=age
>>>
>>data="Gary,50 "
>>d={0:[MyClass(data)]}
>>data="Adam,25 "
>>d[0].append(MyClass (data))
>>>
>>The data is coming from a text file working on a line by line
>>basis. I've just tried and I'm just getting the full string in
>>the first field. That seems logical, now I don't want it to
>>though!

>>
>That's what happens if you use 0 for the key every time. ;)

>
Hmmm... Neil, I may be wrong but I think you didn't get the
point here. As I understand it, Dominic's problem is that it
gets strings like "Gary,50" and would like to call MyClass
initializer this way : MyClass("Gary", "50")

My guess was he doesn't need a class at all, but needed to decide
what he's mapping from->to. It seems far-fetched to me that he
*really* wants a mapping between an index and MyClass objects
containing name and age.

So I tried to cut out the middle-man. Hopefully we can get some
closure on this.

--
Neil Cerutti

**special_dragonfly** · Aug 9 '07, 02:05 PM

Re: Help with Dictionaries and Classes requested please.

"Ben Finney" <bignose+hate s-spam@benfinney. id.auwrote in message
news:87ejic6dkw .fsf@benfinney. id.au...

"special_dragon fly" <Dominic@PLEASE ASK.co.ukwrites :
>

>I've managed to solve the problem, I really was just being a
>dunce.

>
Doubtful; but at this stage we can't tell, because we still don't know
what it is you're actually trying to *do*.
>

>Here's how incase anyone is wondering:
>>
>class MyClass:
> def __init__(self):
> name=""
>dict={}
>dict[0]=[]
>dict[0].append(MyClass ())
>dict[0][0].name="Hello"
>print dict[0][0].name

>
It's not clear why you are using the value 0 for a dictionary key
here; nor why you're assigning an attribute to an object after
creating the object. Neither of them are errors, but without context
it's hard to know what advice to give.
>

The 0 for a key is just an example. The code I actually have would be just
as meaningful at the end of the day. I could have changed MyClass for
class Animals(object) :
def __init__(self, name="", type="", age=""):
self.name=name
self.type=type
self.age=age

dict={'Mouse':[Animals('George ','long eared',20)]}
dict['Mouse'].append(Animals ('Benny','hairy ',30))
dict['Cat']=[Animals('Inigo Montoya','spani sh',10)]

and Neil, Bruno has the right idea of what I was trying to do. However, your
code came in handy still as I used your code elsewhere.see below.

def EnterDictionary (FieldsDictiona ry,key,data):
for i in range(0,int(dat a[6:])):
line=myfile.rea dline()
line=line.strip ()
line=line[6:-1]
if key in FieldsDictionar y:
FieldsDictionar y[key].append(FieldCl ass(*line.split (",")))
else:
FieldsDictionar y[key]=[FieldClass(*lin e.split(","))]

I'd like to thank you all for your patience with me whilst I've asked some
really beginner-like questions. I hope I haven't annoyed you all too much...

In future I would ask however, if it's a really stupid question and you feel
that the answer can be found either by searching google (because in some
cases I don't know what to search for), or in one of the O'reilly books,
just say. In either case, if you could refer me to the search term to use or
the book to read I'd be grateful.

Dominic

**Bruno Desthuilliers** · Aug 9 '07, 03:45 PM

Re: Help with Dictionaries and Classes requested please.

Neil Cerutti a écrit :

On 2007-08-09, Bruno Desthuilliers <bruno.42.desth uilliers@wtf.we bsiteburo.oops. comwrote:

>Neil Cerutti a écrit :

>>On 2007-08-09, special_dragonf ly <Dominic@PLEASE ASK.co.ukwrote:
>>>Is there anyway for python to consider the values within a
>>>string when entering the data into a dictionary. I know that
>>>isn't very clear so here's an example:
>>>>
>>>class MyClass(object) :
>>> def __init__(self,n ame="",age=""):
>>> self.name=name
>>> self.age=age
>>>>
>>>data="Gary,5 0"
>>>d={0:[MyClass(data)]}
>>>data="Adam,2 5"
>>>d[0].append(MyClass (data))
>>>>
>>>The data is coming from a text file working on a line by line
>>>basis. I've just tried and I'm just getting the full string in
>>>the first field. That seems logical, now I don't want it to
>>>though!
>>That's what happens if you use 0 for the key every time. ;)

>Hmmm... Neil, I may be wrong but I think you didn't get the
>point here. As I understand it, Dominic's problem is that it
>gets strings like "Gary,50" and would like to call MyClass
>initializer this way : MyClass("Gary", "50")

>
My guess was he doesn't need a class at all,

Mmm... That's possible (and if all he has in MyClass are name and age
data attributes, then you're obviously right). But then your answer was
perhaps a bit confusing (at least it confused me...)

**Sion Arrowsmith** · Aug 9 '07, 03:55 PM

Re: Help with Dictionaries and Classes requested please.

special_dragonf ly <Dominic@PLEASE ASK.co.ukwrote:

if key in FieldsDictionar y:
FieldsDictionar y[key].append(FieldCl ass(*line.split (",")))
else:
FieldsDictionar y[key]=[FieldClass(*lin e.split(","))]

These four lines can be replaced by:

FieldsDictionar y.setdefault(ke y, []).append(FieldC lass(*line.spli t(",")))

--
\S -- siona@chiark.gr eenend.org.uk -- http://www.chaos.org.uk/~sion/
"Frankly I have no feelings towards penguins one way or the other"
-- Arthur C. Clarke
her nu becomeþ se bera eadward ofdun hlæddre heafdes bæce bump bump bump

**Bruno Desthuilliers** · Aug 9 '07, 04:15 PM

Re: Help with Dictionaries and Classes requested please.

special_dragonf ly a écrit :

"Ben Finney" <bignose+hate s-spam@benfinney. id.auwrote in message
news:87ejic6dkw .fsf@benfinney. id.au...

>"special_drago nfly" <Dominic@PLEASE ASK.co.ukwrites :
>>

>>I've managed to solve the problem, I really was just being a
>>dunce.

>Doubtful; but at this stage we can't tell, because we still don't know
>what it is you're actually trying to *do*.
>>

>>Here's how incase anyone is wondering:
>>>
>>class MyClass:
>> def __init__(self):
>> name=""
>>dict={}
>>dict[0]=[]
>>dict[0].append(MyClass ())
>>dict[0][0].name="Hello"
>>print dict[0][0].name

>It's not clear why you are using the value 0 for a dictionary key
>here; nor why you're assigning an attribute to an object after
>creating the object. Neither of them are errors, but without context
>it's hard to know what advice to give.
>>

The 0 for a key is just an example. The code I actually have would be just
as meaningful at the end of the day. I could have changed MyClass for
class Animals(object) :
def __init__(self, name="", type="", age=""):
self.name=name
self.type=type
self.age=age
>
dict={'Mouse':[Animals('George ','long eared',20)]}
dict['Mouse'].append(Animals ('Benny','hairy ',30))
dict['Cat']=[Animals('Inigo Montoya','spani sh',10)]
>
and Neil, Bruno has the right idea of what I was trying to do. However, your
code came in handy still as I used your code elsewhere.see below.
>
def EnterDictionary (FieldsDictiona ry,key,data):
for i in range(0,int(dat a[6:])):
line=myfile.rea dline()
line=line.strip ()
line=line[6:-1]
if key in FieldsDictionar y:
FieldsDictionar y[key].append(FieldCl ass(*line.split (",")))
else:
FieldsDictionar y[key]=[FieldClass(*lin e.split(","))]

May I suggest a couple possible improvements ?

First : you're of course free to use any naming convention you like, and
it's obviously better to stay consistent, but the canonical Python
convention is to use all_lower for vars, functions (and methods) and
modules, and MixedCase for classes.

About the code now:

def EnterDictionary (FieldsDictiona ry,key,data):
for i in range(0,int(dat a[6:])):

1/ Golden rule : avoid the use of "magic numbers". This one stands true
for any languages !-). The usual solution is to use symbolic constants.
While Python doesn't have real symbolic constant, the convention is to
write them ALL_UPPER.

2/ range() can be used with only one argument, which then will be use as
the upper bound. IOW,
range(0, X)
is the same as
range(X)

line=myfile.rea dline()

3/ where does this 'myfile' comes from ? (hint : don't use globals when
you can avoid them)

line=line.strip ()
line=line[6:-1]

4/ magic numbers again, cf /1. Question : does this 6 has anything to do
with the other one ? What will happen when the file format will change ?

5/ you can do all this in a single line, adding the split() too:
args = myfile.readline ().strip()[XXX:-1].split(",")

if key in FieldsDictionar y:
FieldsDictionar y[key].append(FieldCl ass(*line.split (",")))
else:
FieldsDictionar y[key]=[FieldClass(*lin e.split(","))]

If you expect key to most of the times be already in FieldsDictionna ry,
then a try/except block might be a bit faster. If you expect key to not
be here most of the times, then your solution is right. Note that you
can also use dict.setdefault (key, default):

# probably bad names but I don't have a clue what they should be
DATA_INDEX_OFFS ET = 6
LINE_START = 6
LINE_END = -1

def update_fields_d ict(fields_dict , key, data, datafile):
for i in range(int(data[DATA_INDEX_OFFS ET:])):
args =datafile.readl ine().strip()[LINE_START:LINE _END].split(",")
fields_dict.set default(key, []).append(FieldC lass(*args))

Feel free to take or leave what you consider appropriate here. But by
all means avoid magic numbers, except possibly for Q&D throw-away
scripts (and even then...).

HTH

In future I would ask however, if it's a really stupid question and you feel
that the answer can be found either by searching google (because in some
cases I don't know what to search for), or in one of the O'reilly books,
just say. In either case, if you could refer me to the search term to use or
the book to read I'd be grateful.

That's usually what happens then, don't worry.

**Alex Martelli** · Aug 11 '07, 05:15 AM

Re: Help with Dictionaries and Classes requested please.

Sion Arrowsmith <siona@chiark.g reenend.org.ukw rote:

special_dragonf ly <Dominic@PLEASE ASK.co.ukwrote:

if key in FieldsDictionar y:
FieldsDictionar y[key].append(FieldCl ass(*line.split (",")))
else:
FieldsDictionar y[key]=[FieldClass(*lin e.split(","))]

>
These four lines can be replaced by:
>
FieldsDictionar y.setdefault(ke y, []).append(FieldC lass(*line.spli t(",")))

Even better might be to let FieldsDictionar y be an instance of
collections.def aultdict(list) [[assuming Python 2.5 is in use]], in
which case the simpler

FieldsDictionar y[key].append(FieldCl ass(*line.split (",")))

will Just Work. setdefault was a valiant attempt at fixing this
problem, but defaultdict is better.

Alex

Help with Dictionaries and Classes requested please.

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