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I have the following SQLAlchemy mapped classes:
class User(Base): __tablename__ = "users" email = Column(String, primary_key=True) name = Column(String) class Document(Base): __tablename__ = "documents" name = Column(String, primary_key=True) author = Column(String, ForeignKey("users.email")) class DocumentsPermissions(Base): __tablename__ = "documents_permissions" readAllowed = Column(Boolean) writeAllowed = Column(Boolean) document = Column(String, ForeignKey("documents.name"))
I need to get a table like this for
user.email = "[email protected]":
email | name | document_name | document_readAllowed | document_writeAllowed
How can it be made using one query request for SQLAlchemy? The code below does not work for me:
result = session.query(User, Document, DocumentPermission).filter_by(email = "[email protected]").all()
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sqlalchemy: how to join several tables by one query? filter: Questions
List comprehension vs. lambda + filter
I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items.
My code looked like this:
my_list = [x for x in my_list if x.attribute == value]
But then I thought, wouldn"t it be better to write it like this?
my_list = filter(lambda x: x.attribute == value, my_list)
It"s more readable, and if needed for performance the lambda could be taken out to gain something.
Question is: are there any caveats in using the second way? Any performance difference? Am I missing the Pythonic Way‚Ñ¢ entirely and should do it in yet another way (such as using itemgetter instead of the lambda)?
It is strange how much beauty varies for different people. I find the list comprehension much clearer than
lambda, but use whichever you find easier.
There are two things that may slow down your use of
The first is the function call overhead: as soon as you use a Python function (whether created by
lambda) it is likely that filter will be slower than the list comprehension. It almost certainly is not enough to matter, and you shouldn"t think much about performance until you"ve timed your code and found it to be a bottleneck, but the difference will be there.
The other overhead that might apply is that the lambda is being forced to access a scoped variable (
value). That is slower than accessing a local variable and in Python 2.x the list comprehension only accesses local variables. If you are using Python 3.x the list comprehension runs in a separate function so it will also be accessing
value through a closure and this difference won"t apply.
The other option to consider is to use a generator instead of a list comprehension:
def filterbyvalue(seq, value): for el in seq: if el.attribute==value: yield el
Then in your main code (which is where readability really matters) you"ve replaced both list comprehension and filter with a hopefully meaningful function name.
This is a somewhat religious issue in Python. Even though Guido considered removing
reduce from Python 3, there was enough of a backlash that in the end only
reduce was moved from built-ins to functools.reduce.
Personally I find list comprehensions easier to read. It is more explicit what is happening from the expression
[i for i in list if i.attribute == value] as all the behaviour is on the surface not inside the filter function.
I would not worry too much about the performance difference between the two approaches as it is marginal. I would really only optimise this if it proved to be the bottleneck in your application which is unlikely.
Also since the BDFL wanted
filter gone from the language then surely that automatically makes list comprehensions more Pythonic ;-)
sqlalchemy: how to join several tables by one query? filter: Questions
How do I do a not equal in Django queryset filtering?
In Django model QuerySets, I see that there is a
__lt for comparative values, but is there a
!= (not equals)? I want to filter out using a not equals. For example, for
Model: bool a; int x;
I want to do
results = Model.objects.exclude(a=True, x!=5)
!= is not correct syntax. I also tried
I ended up using:
results = Model.objects.exclude(a=True, x__lt=5).exclude(a=True, x__gt=5)
from myapp.models import Entry from django.db.models import Q Entry.objects.filter(~Q(id=3))
will return all entries except the one(s) with
3 as their ID:
[<Entry: Entry object>, <Entry: Entry object>, <Entry: Entry object>, ...]
Why is it string.join(list) instead of list.join(string)?
This has always confused me. It seems like this would be nicer:
my_list = ["Hello", "world"] print(my_list.join("-")) # Produce: "Hello-world"
my_list = ["Hello", "world"] print("-".join(my_list)) # Produce: "Hello-world"
Is there a specific reason it is like this?
It"s because any iterable can be joined (e.g, list, tuple, dict, set), but its contents and the "joiner" must be strings.
"_".join(["welcome", "to", "stack", "overflow"]) "_".join(("welcome", "to", "stack", "overflow"))
Using something other than strings will raise the following error:
TypeError: sequence item 0: expected str instance, int found
This was discussed in the String methods... finally thread in the Python-Dev achive, and was accepted by Guido. This thread began in Jun 1999, and
str.join was included in Python 1.6 which was released in Sep 2000 (and supported Unicode). Python 2.0 (supported
str methods including
join) was released in Oct 2000.
- There were four options proposed in this thread:
joinas a built-in function
- Guido wanted to support not only
tuples, but all sequences/iterables.
seq.reduce(str)is difficult for newcomers.
seq.join(str)introduces unexpected dependency from sequences to str/unicode.
join()as a built-in function would support only specific data types. So using a built-in namespace is not good. If
join()supports many datatypes, creating an optimized implementation would be difficult, if implemented using the
__add__method then it would ve
- The separator string (
sep) should not be omitted. Explicit is better than implicit.
Here are some additional thoughts (my own, and my friend"s):
- Unicode support was coming, but it was not final. At that time UTF-8 was the most likely about to replace UCS2/4. To calculate total buffer length of UTF-8 strings it needs to know character coding rule.
- At that time, Python had already decided on a common sequence interface rule where a user could create a sequence-like (iterable) class. But Python didn"t support extending built-in types until 2.2. At that time it was difficult to provide basic
iterableclass (which is mentioned in another comment).
Guido"s decision is recorded in a historical mail, deciding on
Funny, but it does seem right! Barry, go for it...
Guido van Rossum
join() method is in the string class, instead of the list class?
I agree it looks funny.
Historical note. When I first learned Python, I expected join to be a method of a list, which would take the delimiter as an argument. Lots of people feel the same way, and there‚Äôs a story behind the join method. Prior to Python 1.6, strings didn‚Äôt have all these useful methods. There was a separate string module which contained all the string functions; each function took a string as its first argument. The functions were deemed important enough to put onto the strings themselves, which made sense for functions like lower, upper, and split. But many hard-core Python programmers objected to the new join method, arguing that it should be a method of the list instead, or that it shouldn‚Äôt move at all but simply stay a part of the old string module (which still has lots of useful stuff in it). I use the new join method exclusively, but you will see code written either way, and if it really bothers you, you can use the old string.join function instead.
--- Mark Pilgrim, Dive into Python