I need a bit of Python refactoring advice.
I have a list of dict objects (new_monitors), which can be empty. When there are new monitors, however, I want to add a bunch of fields to those monitors.
For each monitor, I would like to append all not None fields from the DogDump.HIDE_FIELDS dict:
if new_monitors:
for monitor in new_monitors:
for key, value in DogDump.HIDE_FIELDS.items():
if value:
monitor[key] = value
Note: This snippet below worked very well, but it included all of the None fields. I do not want the None fields!
if new_monitors:
for monitor in new_monitors:
monitor.update(DogDump.HIDE_FIELDS)
How can I refactor this snippet that looks more pythonic, but still maintain good readability?
Not sure which is really the most "pythonic" way of handling your need to filter DogDump.HIDE_FIELDS dict before adding the relevant key / value pairs to your monitor dict. One way would be to perform the "filtering" with dict comprehension.
Also, I would think that you could "filter" your DogDump.HIDE_FIELDS dict before your loop rather than repeating this operation for each loop iteration (unless there are other operations taking place that mutate DogDump.HIDE_FIELDS while you are iterating).
Example of "filtering" with dict comprehension (dump refers to your DogDump.HIDE_FIELDS dict):
monitor = {'key': 'value'}
dump = {'a': 1, 'b': None}
dump_filtered = {k:v for (k,v) in dump.items() if v}
monitor.update(dump_filtered)
print(monitor)
# OUTPUT
# {'key': 'value', 'a': 1}
I have a dictionary that looks like:
{u'message': u'Approved', u'reference': u'A71E7A739E24', u'success': True}
I would like to retrieve the key-value pair for reference, i.e. { 'reference' : 'A71E7A739E24' }.
I'm trying to do this using iteritems which does return k, v pairs, and then I'm adding them to a new dictionary. But then, the resulting value is unicode rather than str for some reason and I'm not sure if this is the most straightforward way to do it:
ref = {}
for k, v in charge.iteritems():
if k == 'reference':
ref['reference'] = v
print ref
{'reference': u'A71E7A739E24'}
Is there a built-in way to do this more easily? Or, at least, to avoid using iteritems and simply return:
{ 'reference' : 'A71E7A739E24' }
The trouble with using iteritems is that you increase lookup time to O(n) where n is dictionary size, because you are no longer using a hash table
If you only need to get one key-value pair, it's as simple as
ref = { key: d[key] }
If there may be multiple pairs that are selected by some condition,
either use dict from iterable constructor (the 2nd version is better if your condition depends on values, too):
ref = dict(k,d[k] for k in charge if <condition>)
ref = dict(k,v for k,v in charge.iteritems() if <condition>)
or (since 2.7) a dict comprehension (which is syntactic sugar for the above):
ref = {k,d[k] for k in charge if <condition>}
<same as above>
I dont understand the question:
is this what you are trying to do:
ref={'reference',charge["reference"]}
I am pretty new to Python (coming from a Java background) so was wondering if someone would have any advice on a data structure design question. I need to create a data structure with default values that would look something like this:
[
(Name=”name1”, {id=1, val1=”val1”} ),
(Name=”name2”, {id=2, val1=”val2”} )
]
i.e a list of tuples where each tuple consists of one string value (Name) and a dictionary of values.
The first piece of functionality I need is to be able to add to or override the above data structure with additional details e.g:
[
(Name=”name2”, {id=2, val1=”new value”} ) ,
(Name=”name2”, {id=3, val1=”another value”} ) ,
(Name=”name3”, {id=3, val1=”val3”} )
]
Which would ultimately result in a final data structure that looks like this:
[
(Name=”name1”, {id=1, val1=”val1”} ),
(Name=”name2”, {id=2, val1=”new value”} ) ,
(Name=”name2”, {id=3, val1=”another value”} ) ,
(Name=”name3”, {id=3, val1=”val3”} )
]
The second piece of functionality I need is to be able to access each tuple in the list according to the id value in the dictionary i.e
Get me tuple where name = “name2” and id=”3” .
Could anybody give me their opinions on how best this could be implemented in Python?
Thanks!
The namedtuple is closest to what you wrote but as others have said, there may be better designs for what you want.
You should try using dictionaries may be.
A dictionary is mutable and is another container type that can store any number of Python objects, including other container types. Dictionaries consist of pairs (called items) of keys and their corresponding values.
Python dictionaries are also known as associative arrays or hash tables. The general syntax of a dictionary is as follows:
dict = {'Alice': '2341', 'Beth': '9102', 'Cecil': '3258'}
You can create dictionary in the following way as well:
dict1 = { 'abc': 456 };
dict2 = { 'abc': 123, 98.6: 37 };
Each key is separated from its value by a colon (:), the items are separated by commas, and the whole thing is enclosed in curly braces. An empty dictionary without any items is written with just two curly braces, like this: {}.
Keys are unique within a dictionary while values may not be. The values of a dictionary can be of any type, but the keys must be of an immutable data type such as strings, numbers, or tuples.
Accessing Values in Dictionary:
To access dictionary elements, you can use the familiar square brackets along with the key to obtain its value. Following is a simple example:
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
print "dict['Name']: ", dict['Name'];
print "dict['Age']: ", dict['Age'];
When the above code is executed, it produces the following result:
dict['Name']: Zara
dict['Age']: 7
Source: http://www.tutorialspoint.com/python/python_dictionary.htm
Consider a dict like
mydict = {
'Apple': {'American':'16', 'Mexican':10, 'Chinese':5},
'Grapes':{'Arabian':'25','Indian':'20'} }
How do I access for instance a particular element of this dictionary?
for instance, I would like to print the first element after some formatting the first element of Apple which in our case is 'American' only?
Additional information
The above data structure was created by parsing an input file in a python function. Once created however it remains the same for that run.
I am using this data structure in my function.
So if the file changes, the next time this application is run the contents of the file are different and hence the contents of this data structure will be different but the format would be the same.
So you see I in my function I don't know that the first element in Apple is 'American' or anything else so I can't directly use 'American' as a key.
Given that it is a dictionary you access it by using the keys. Getting the dictionary stored under "Apple", do the following:
>>> mydict["Apple"]
{'American': '16', 'Mexican': 10, 'Chinese': 5}
And getting how many of them are American (16), do like this:
>>> mydict["Apple"]["American"]
'16'
If the questions is, if I know that I have a dict of dicts that contains 'Apple' as a fruit and 'American' as a type of apple, I would use:
myDict = {'Apple': {'American':'16', 'Mexican':10, 'Chinese':5},
'Grapes':{'Arabian':'25','Indian':'20'} }
print myDict['Apple']['American']
as others suggested. If instead the questions is, you don't know whether 'Apple' as a fruit and 'American' as a type of 'Apple' exist when you read an arbitrary file into your dict of dict data structure, you could do something like:
print [ftype['American'] for f,ftype in myDict.iteritems() if f == 'Apple' and 'American' in ftype]
or better yet so you don't unnecessarily iterate over the entire dict of dicts if you know that only Apple has the type American:
if 'Apple' in myDict:
if 'American' in myDict['Apple']:
print myDict['Apple']['American']
In all of these cases it doesn't matter what order the dictionaries actually store the entries. If you are really concerned about the order, then you might consider using an OrderedDict:
http://docs.python.org/dev/library/collections.html#collections.OrderedDict
As I noticed your description, you just know that your parser will give you a dictionary that its values are dictionary too like this:
sampleDict = {
"key1": {"key10": "value10", "key11": "value11"},
"key2": {"key20": "value20", "key21": "value21"}
}
So you have to iterate over your parent dictionary. If you want to print out or access all first dictionary keys in sampleDict.values() list, you may use something like this:
for key, value in sampleDict.items():
print value.keys()[0]
If you want to just access first key of the first item in sampleDict.values(), this may be useful:
print sampleDict.values()[0].keys()[0]
If you use the example you gave in the question, I mean:
sampleDict = {
'Apple': {'American':'16', 'Mexican':10, 'Chinese':5},
'Grapes':{'Arabian':'25','Indian':'20'}
}
The output for the first code is:
American
Indian
And the output for the second code is:
American
EDIT 1:
Above code examples does not work for version 3 and above of python; since from version 3, python changed the type of output of methods keys and values from list to dict_values. Type dict_values is not accepting indexing, but it is iterable. So you need to change above codes as below:
First One:
for key, value in sampleDict.items():
print(list(value.keys())[0])
Second One:
print(list(list(sampleDict.values())[0].keys())[0])
I know this is 8 years old, but no one seems to have actually read and answered the question.
You can call .values() on a dict to get a list of the inner dicts and thus access them by index.
>>> mydict = {
... 'Apple': {'American':'16', 'Mexican':10, 'Chinese':5},
... 'Grapes':{'Arabian':'25','Indian':'20'} }
>>>mylist = list(mydict.values())
>>>mylist[0]
{'American':'16', 'Mexican':10, 'Chinese':5},
>>>mylist[1]
{'Arabian':'25','Indian':'20'}
>>>myInnerList1 = list(mylist[0].values())
>>>myInnerList1
['16', 10, 5]
>>>myInnerList2 = list(mylist[1].values())
>>>myInnerList2
['25', '20']
As a bonus, I'd like to offer kind of a different solution to your issue. You seem to be dealing with nested dictionaries, which is usually tedious, especially when you have to check for existence of an inner key.
There are some interesting libraries regarding this on pypi, here is a quick search for you.
In your specific case, dict_digger seems suited.
>>> import dict_digger
>>> d = {
'Apple': {'American':'16', 'Mexican':10, 'Chinese':5},
'Grapes':{'Arabian':'25','Indian':'20'}
}
>>> print(dict_digger.dig(d, 'Apple','American'))
16
>>> print(dict_digger.dig(d, 'Grapes','American'))
None
You can use mydict['Apple'].keys()[0] in order to get the first key in the Apple dictionary, but there's no guarantee that it will be American. The order of keys in a dictionary can change depending on the contents of the dictionary and the order the keys were added.
You can't rely on order of dictionaries, but you may try this:
mydict['Apple'].items()[0][0]
If you want the order to be preserved you may want to use this:
http://www.python.org/dev/peps/pep-0372/#ordered-dict-api
Simple Example to understand how to access elements in the dictionary:-
Create a Dictionary
d = {'dog' : 'bark', 'cat' : 'meow' }
print(d.get('cat'))
print(d.get('lion'))
print(d.get('lion', 'Not in the dictionary'))
print(d.get('lion', 'NA'))
print(d.get('dog', 'NA'))
Explore more about Python Dictionaries and learn interactively here...
Few people appear, despite the many answers to this question, to have pointed out that dictionaries are un-ordered mappings, and so (until the blessing of insertion order with Python 3.7) the idea of the "first" entry in a dictionary literally made no sense. And even an OrderedDict can only be accessed by numerical index using such uglinesses as mydict[mydict.keys()[0]] (Python 2 only, since in Python 3 keys() is a non-subscriptable iterator.)
From 3.7 onwards and in practice in 3,6 as well - the new behaviour was introduced then, but not included as part of the language specification until 3.7 - iteration over the keys, values or items of a dict (and, I believe, a set also) will yield the least-recently inserted objects first. There is still no simple way to access them by numerical index of insertion.
As to the question of selecting and "formatting" items, if you know the key you want to retrieve in the dictionary you would normally use the key as a subscript to retrieve it (my_var = mydict['Apple']).
If you really do want to be able to index the items by entry number (ignoring the fact that a particular entry's number will change as insertions are made) then the appropriate structure would probably be a list of two-element tuples. Instead of
mydict = {
'Apple': {'American':'16', 'Mexican':10, 'Chinese':5},
'Grapes':{'Arabian':'25','Indian':'20'} }
you might use:
mylist = [
('Apple', {'American':'16', 'Mexican':10, 'Chinese':5}),
('Grapes', {'Arabian': '25', 'Indian': '20'}
]
Under this regime the first entry is mylist[0] in classic list-endexed form, and its value is ('Apple', {'American':'16', 'Mexican':10, 'Chinese':5}). You could iterate over the whole list as follows:
for (key, value) in mylist: # unpacks to avoid tuple indexing
if key == 'Apple':
if 'American' in value:
print(value['American'])
but if you know you are looking for the key "Apple", why wouldn't you just use a dict instead?
You could introduce an additional level of indirection by cacheing the list of keys, but the complexities of keeping two data structures in synchronisation would inevitably add to the complexity of your code.
With the following small function, digging into a tree-shaped dictionary becomes quite easy:
def dig(tree, path):
for key in path.split("."):
if isinstance(tree, dict) and tree.get(key):
tree = tree[key]
else:
return None
return tree
Now, dig(mydict, "Apple.Mexican") returns 10, while dig(mydict, "Grape") yields the subtree {'Arabian':'25','Indian':'20'}. If a key is not contained in the dictionary, dig returns None.
Note that you can easily change (or even parameterize) the separator char from '.' to '/', '|' etc.
mydict = {
'Apple': {'American':'16', 'Mexican':10, 'Chinese':5},
'Grapes':{'Arabian':'25','Indian':'20'} }
for n in mydict:
print(mydict[n])
Here is my problem: I have a list of Python dictionaries of identical form, that are meant to represent the rows of a table in a database, something like this:
[ {'ID': 1,
'NAME': 'Joe',
'CLASS': '8th',
... },
{'ID': 1,
'NAME': 'Joe',
'CLASS': '11th',
... },
...]
I have already written a function to get the unique values for a particular field in this list of dictionaries, which was trivial. That function implements something like:
select distinct NAME from ...
However, I want to be able to get the list of multiple unique fields, similar to:
select distinct NAME, CLASS from ...
Which I am finding to be non-trivial. Is there an algorithm or Python included function to help me with this quandry?
Before you suggest loading the CSV files into a SQLite table or something similar, that is not an option for the environment I'm in, and trust me, that was my first thought.
If you want it as a generator:
def select_distinct(dictionaries, keys):
seen = set()
for d in dictionaries:
v = tuple(d[k] for k in keys)
if v in seen: continue
yield v
seen.add(v)
if you want the result in some other form (e.g., a list instead of a generator) it's not hard to alter this (e.g., .append to the initially-empty result list instead of yielding, and return the result list at the end).
To be called, of course, as
for values_tuple in select_distinct(thedicts, ('NAME', 'CLASS')):
...
or the like.
distinct_list = list(set([(d['NAME'], d['CLASS']) for d in row_list]))
where row_list is the list of dicts you have
One can implement the task using hashing. Just hash the content of rows that appear in the distinct query and ignore the ones with same hash.