Background
I have a module called db.py that is basically consist of wrapper functions that make calls to the db. I have a table called nba and that has columns like player_name age player_id etc.
I have a simple function called db_cache() where i make a call to the db table and request to get all the player ids. The output of the response looks something like this
[Record(player_id='31200952409069'), Record(player_id='31201050710077'), Record(player_id='31201050500545'), Record(player_id='31001811412442'), Record(player_id='31201050607711')]
Then I simply iterate through the list and dump each item inside a dictionary.
I am wondering if there is a more pythonic way to populate the dictionary?
My code
def db_cache():
my_dict: Dict[str, None] = {}
response = db.run_query(sql="SELECT player_id FROM nba")
for item in response:
my_dict[item.player_id] = None
return my_dict
my_dict = db_cache()
This is built-in to the dict type:
>>> help(dict.fromkeys)
Help on built-in function fromkeys:
fromkeys(iterable, value=None, /) method of builtins.type instance
Create a new dictionary with keys from iterable and values set to value.
The value we want is the default of None, so all we need is:
my_dict = dict.from_keys(db.run_query(sql="SELECT player_id FROM nba"))
Note that the value will be reused, and not copied, which can cause problems if you want to use a mutable value. In these cases, you should instead simply use the dict comprehension, as given in #AvihayTsayeg's answer.
my_arr = [1,2,3,4]
my_dict = {"item":item for item in my_arr}
I am new to django so apologies if this is not possible or easy.
I have a view that takes a subset of a model
data = Terms.objects.filter(language = language_id)
The subset is one language. The set has a number of concepts for a language. Some languages might use the same word for multiple concepts, and I want to colour these the same in an SVG image. So I do this next:
for d in data:
if d.term is None:
d.colour = "#D3D3D3"
else:
d.colour = termColours[d.term]
Where termColours is a dictionary with keys as the unique terms and values as the hexadecimal colour I want.
I thought this would add a new colour attribute to my queryset. However, when I convert the queryset to json (in order to pass it to JS) the colour object is not there.
terms_json = serializers.serialize('json', data)
How can I add a new colour element to my queryset?
Convert your Queryset to Dict and then modify values.
Ex:
data = Terms.objects.filter(language = language_id).values()
for d in data:
if d.term is None:
d.colour = "#D3D3D3"
else:
d.colour = termColours[d.term]
If I understand correctly - you need Django ORM annotation. And it might look like that:
from django.db.models import Case, When, Value
data = Terms.objects.filter(language = language_id)
.annotate(color = Case(
When(term__isnull = True, then = "#D3D3D3"),
When(term__isnull = False, then = termColours[Value(term)]),))
Only problem here - I don't exactly know this moment - termColours[Value(term)], you need to test different combinations of that expressions to get the value of field term.
I'm trying to query an NDB model using a list of provided key id strings. The model has string ids that are assigned at creation - for example:
objectKey = MyModel(
id="123456ABC",
name="An Object"
).put()
Now I can't figure out how to query the NDB key ids with a list filter. Normally you can do the MyModel.property.IN() to query properties:
names = ['An Object', 'Something else', 'etc']
# This query works
query = MyModel.query(MyModel.name.IN(names))
When I try to filter by a list of keys, I can't get it to work:
# This simple get works
object = MyModel.get_by_id("123456ABC")
ids = ["123456ABC", "CBA654321", "etc"]
# These queries DON'T work
query = MyModel.query(MyModel.id.IN(ids))
query = MyModel.query(MyModel.key.id.IN(ids))
query = MyModel.query(MyModel.key.id().IN(ids))
query = MyModel.query(MyModel._properties['id'].IN(ids))
query = MyModel.query(getattr(MyModel, 'id').IN(ids))
...
I always get AttributeError: type object 'MyModel' has no attribute 'id' errors.
I need to be able to filter by a list of IDs, rather than iterate through each ID in the list (which is sometimes long). How do I do it?
The following should work:
keys = [ndb.Key(MyModel, anid) for anid in ids]
objs = ndb.get_multi(keys)
You can also use urlsafe keys If you have problems using the ids.
keys = ndb.get_multi([ndb.Key(urlsafe=k) for k in ids])
I would like to use the in_ operator in sqlalchemy using two values, one of them being NULL (mysql NULL), I don't know how to pass it via Python?
So I have a Python cgi that contains a bunch of parameters that I format then finally store inside a dict queryValues (the key being the column name and the value being a value sent by the user stored inside a fieldStorage)
for attr,value in queryValues.items() : #queryValues is a dict of parameters
valueWithNone = value.append(None) #I want to includ NULL
and_args_of_cr = [(and_(getattr(TableCR.c,attr).in_(valueWithNone)))]
I tried None and sqlalchemy.sql.null(), also tried putting directly in_(value,None) but value has the form ['Yes'] so I don't know how to do this.
But it's not working, how can I do this please?
The line value.append(None) is an in-place modification and does not return anything, so valueWithNone will be None. This is probably what you're after:
for attr,value in queryValues.items():
queryvalues = value[:] # Create a copy of list
queryvalues.append(None)
and_args_of_cr = [(and_(getattr(TableCR.c,attr).in_(queryvalues)))]
I have the following code:
query = """
SELECT Coalesce((SELECT sp.param_value
FROM sites_params sp
WHERE sp.param_name = 'ci'
AND sp.site_id = s.id
ORDER BY sp.id DESC
LIMIT 1), -1) AS ci
FROM sites s
WHERE s.deleted = 0
AND s.id = 10
"""
site = db_session.execute(query)
# print site
# <sqlalchemy.engine.result.ResultProxy object at 0x033E63D0>
site = db_session.execute(query).fetchone()
print site # (u'375')
print list(site) # [u'375']
Why does SQLAlchemy return tuples, not dicts, for this query? I want to use the following style to access the results of the query:
print site.ci
# u'375'
This is an old question, but still relevant today. Getting SQL Alchemy to return a dictionary is very useful, especially when working with RESTful based APIs that return JSON.
Here is how I did it using the db_session in Python 3:
resultproxy = db_session.execute(query)
d, a = {}, []
for rowproxy in resultproxy:
# rowproxy.items() returns an array like [(key0, value0), (key1, value1)]
for column, value in rowproxy.items():
# build up the dictionary
d = {**d, **{column: value}}
a.append(d)
The end result is that the array a now contains your query results in dictionary format.
As for how this works in SQL Alchemy:
Thedb_session.execute(query) returns a ResultProxy object
The ResultProxy object is made up of RowProxy objects
The RowProxy object has an .items() method that returns key, value tuples of all the items in the row, which can be unpacked as key, value in a for operation.
And here a one-liner alternative:
[{column: value for column, value in rowproxy.items()} for rowproxy in resultproxy]
From the docs:
class sqlalchemy.engine.RowProxy(parent, row, processors, keymap)
Proxy values from a single cursor row.
Mostly follows “ordered dictionary” behavior, mapping result values to the string-based column name, the integer position of the result in the row, as well as Column instances which can be mapped to the original Columns that produced this result set (for results that correspond to constructed SQL expressions).
has_key(key)
Return True if this RowProxy contains the given key.
items()
Return a list of tuples, each tuple containing a key/value pair.
keys()
Return the list of keys as strings represented by this RowProxy.
Link: http://docs.sqlalchemy.org/en/latest/core/connections.html#sqlalchemy.engine.RowProxy.items
Did you take a look at the ResultProxy docs?
It describes exactly what #Gryphius and #Syed Habib M suggest, namely to use site['ci'].
The ResultProxy does not "return a tuple" as you claim - it is (not surprisingly) a proxy that behaves (e.g. prints) like a tuple but also supports dictionary-like access:
From the docs:
Individual columns may be accessed by their integer position,
case-insensitive column name, or by schema.Column object. e.g.:
row = fetchone()
col1 = row[0] # access via integer position
col2 = row['col2'] # access via name
col3 = row[mytable.c.mycol] # access via Column object.
I've built a simple class to work like a database interface in our processes. Here it goes:
from sqlalchemy import create_engine
class DBConnection:
def __init__(self, db_instance):
self.db_engine = create_engine('your_database_uri_string')
self.db_engine.connect()
def read(self, statement):
"""Executes a read query and returns a list of dicts, whose keys are column names."""
data = self.db_engine.execute(statement).fetchall()
results = []
if len(data)==0:
return results
# results from sqlalchemy are returned as a list of tuples; this procedure converts it into a list of dicts
for row_number, row in enumerate(data):
results.append({})
for column_number, value in enumerate(row):
results[row_number][row.keys()[column_number]] = value
return results
You can easily convert each result row to a dictionary by using dict(site).
Then site['ci'] would be available if ci column is exists.
In order to have site.ci (according to https://stackoverflow.com/a/22084672/487460):
from collections import namedtuple
Site = namedtuple('Site', site.keys())
record = Site(*site)
This may help solve the OPs question. I think the problem he was having is that the row object only contained column values, but not the column names themselves, as is the case with ORM queries where the results have a dict attribute with both keys and values.
python sqlalchemy get column names dynamically?
The easiest way that I found is using list comprehension with calling dict() func on every RowProxy:
site = db_session.execute(query)
result = [dict(row) for row in site]
Based on Essential SQLAlchemy book:
A ResultProxy is a wrapper around a DBAPI cursor object, and its main
goal is to make it easier to use and manipulate the results of a
statement
Simple select example:
from sqlalchemy.sql import select
stmnt = select([cookies])
result_proxy = connection.execute(stmnt)
results = result_proxy.fetchall()
Results going to be like this:
# ID, cookie_name, quantity, amount
[
(1, u'chocolate chip', 12, Decimal('0.50')),
(2, u'dark chocolate chip', 1, Decimal('0.75')),
(3, u'peanut butter', 24, Decimal('0.25')),
(4, u'oatmeal raisin', 100, Decimal('1.00'))
]
It makes handling query results easier by allowing access using an index, name, or Column object.
Accessing cookie_name in different ways:
first_row = results[0]
first_row[1]
first_row.cookie_name
first_row[cookies.c.cookie_name]
These all result in u'chocolate chip' and they each reference the exact same data element in the first record of our results variable. This flexibility in access is only part of the power of the ResultProxy.
We can also leverage the ResultProxy as an iterable:
result_proxy = connection.execute(stmnt)
for record in result_proxy:
print(record.cookie_name)
This method uses list comprehensions, it receives a sql alchemy rowset object and returns the same items as a list of dictionaries:
class ResultHelper():
#classmethod
def resultproxy_to_list(cls, sql_alchemy_rowset):
return [{tuple[0]: tuple[1] for tuple in rowproxy.items()}
for rowproxy in sql_alchemy_rowset]
As you call db.execute(sql).fetchall(), you can easily use the following function to parse the return data to a dict:
def query_to_dict(ret):
if ret is not None:
return [{key: value for key, value in row.items()} for row in ret if row is not None]
else:
return [{}]