[Tutor] confusions about re module
someone please tell me why i'm getting this output?
specially the 'e3%' ! ! !
>>> import re
>>> re.findall('([\w]+.)','abdd.e3\45 dret.8dj st.jk')
['abdd.', 'e3%', 'dret.', '8dj ', 'st.', 'jk']
I am getting the same output for the following too..
>>> re.findall(r'([\w]+.)','abdd.e3\45 dret.8dj st.jk')
['abdd.', 'e3%', 'dret.', '8dj ', 'st.', 'jk']
wasn't i supposed to get ['abdd.','dret.'] ??
python version: 2.6.5
os: windows
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[Tutor] Combining two Dictionaries
Hello everyone,
I had an interesting thing come up earlier in my programming, and I'm trying
to wrap my mind around why it occurred.
I wanted to take two dictionaries with the same keys and combine their
values to make one, big super dictionary.
def combine(d1, d2):
for key in d1:
if key in d2:
d2[key] += d1[key]
When I assign values to each dictionary, this works perfectly.
d1 = {'a': 1, 'b': 2, 'c': 3}
d2 = {'a': 10, 'b': 20, 'c': 30}
combine(d1, d2)
d1 = {'a': 1, 'b': 2, 'c': 3}
d2 = {'a': 11, 'b': 22, 'c': 33}
When I initialize the class which holds these dictionaries, though, I need
to make sure that all the keys contained in d2 match the keys of d1. Thus I
tried:
d1 = {'a': 0, 'b': 0, 'c': 0}
d2 = d1
My understanding was that d2 looked at d1 once, grabbed its keys and values,
and went off to do its own thing. Just as if you typed:
x = 3
y = x
x = 6
y still holds the value 3.
This turns out not to be the case with dictionaries, and I'm not sure why
this is so. Why when you change a dictionary's keys in place does a copied
dictionary take on the new values?
Thanks for any help you can provide.
Best,
Ryan
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Re: [Tutor] Combining two Dictionaries
def combine(d1, d2):
for key in d1:
if key in d2:
d2[key] += d1[key]
[Remark]
I usually avoid changing function arguments. But later on you're talking
about using this function as a method a class, so d1 and d2 would be
instance attributes I guess.
When I initialize the class which holds these dictionaries, though, I need
to make sure that all the keys contained in d2 match the keys of d1. Thus I
tried:
d1 = {'a': 0, 'b': 0, 'c': 0}
Now d1 holds the address of a dict in memory. The dict contains your data.
d2 = d1
Now d2 holds the same address of the same dict in memory. d1 and d2
refer to the same dictionary.
My understanding was that d2 looked at d1 once, grabbed its keys and values,
and went off to do its own thing.
You would need to use copy() explicitly.
import copy
d2 = copy.copy(d1) # swallow copy
# or a deep copy
# -> copies also referenced objects recursively
# usefull if you have e.g. lists as values in your dict
# and want them copied too.
d2 = copy.deepcopy(d1)
Just as if you typed:
x = 3
y = x
x = 6
y still holds the value 3.
Python has mutable and immutable types. Str and int, for example, are
immutable. Dicts and list for example are mutable.
The statement x=6 binds x to a new object, because the object with value
3 cannot be changed.
There are a lot of explanations about mutable and immutable types. Much
better then I could. You could search the list, for example here:
http://dir.gmane.org/gmane.comp.python.tutor
or the web or see this overview in the docs:
http://docs.python.org/reference/datamodel.html#objects-values-and-types
Generally, I wouldn't copy an existing dict to make sure the other dict
has the same values. The data must come from somewhere. I would check
the data, then you wouldn't need to check the container.
Finally, you could use dict.get with a default set to 0, then you
wouldn't need to care, e.g. (using dict comprehension and creating a new
dict):
def combine_2(d1, d2):
return {key: d1[key]+d2.get(key, 0) for key in d1}
But that's a bit dirty semantic wise, better to check the input data ...
just to show you the syntax.
HTH,
Jan
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Re: [Tutor] Combining two Dictionaries
On Sun, May 1, 2011 at 5:49 AM, Ryan Strunk wrote:
> When I initialize the class which holds these dictionaries, though, I need
> to make sure that all the keys contained in d2 match the keys of d1. Thus I
> tried:
> d1 = {'a': 0, 'b': 0, 'c': 0}
> d2 = d1
> My understanding was that d2 looked at d1 once, grabbed its keys and values,
> and went off to do its own thing. Just as if you typed:
> x = 3
> y = x
> x = 6
> y still holds the value 3.
> This turns out not to be the case with dictionaries, and I'm not sure why
> this is so. Why when you change a dictionary's keys in place does a copied
> dictionary take on the new values?
> Thanks for any help you can provide.
To answer your question, we have to look at Python's data model, which
differs from that in other languages. What
y = x
does, is to calculate the object x and give it the name y, apart from
whatever names it may or may not have already. Thus, it does _not_
make a copy, but x and y are two different names of the _same_ object.
What happens in your example, is that you give the object 3 the name
x, then give the object 3 (the outcome of the 'calculation' x) the
name y, then give the object 6 the name x. After that y is still a
name for 3. But if you're working with dictionaries, you're probably
doing something like this:
d1 = {'a': 0, 'b': 0, 'c': 0} # You create a dictionary and give it the name d1
d2 = d1 # You give _the same_ dictionary the name d2
d1['c'] = 1 # You _change_ the dictionary by changing its value for
the key c. d1 and d2 still are names for the _same_ dictionary
Just like in the integer situation, d2 is still a name for the same
object as it was before, but this time _the object itself has
changed_. And d2 'sees' the change of the object.
--
André Engels, andreeng...@gmail.com
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