According to the docs, your second attempt should work:

&gt; To refer to models defined in another application, you must instead explicitly specify the application label. For example, if the Manufacturer model above is defined in another application called production, you&#39;d need to use:


    class Car(models.Model):
        manufacturer = models.ForeignKey(&#39;production.Manufacturer&#39;)

Have you tried putting it into quotes?

It is also possible to pass the class itself:

    from django.db import models
    from production import models as production_models

    class Car(models.Model):
        manufacturer = models.ForeignKey(production_models.Manufacturer)

OK - I&#39;ve figured it out. You can do it, you just have to use the right `import` syntax. The correct syntax is:

    from prototype.cf.models import Movie

My mistake was not specifying the `.models` part of that line. D&#39;oh!


If I have a function that needs to work with a `shared_ptr`, wouldn&#39;t it be more efficient to pass it a reference to it (so to avoid copying the `shared_ptr` object)?
What are the possible bad side effects?
I envision two possible cases:

1) inside the function a copy is made of the argument, like in

    ClassA::take_copy_of_sp(boost::shared_ptr&lt;foo&gt; &amp;sp)  
    {  
         ...  
         m_sp_member=sp; //This will copy the object, incrementing refcount  
         ...  
    }  

2) inside the function the argument is only used, like in  
    
    Class::only_work_with_sp(boost::shared_ptr&lt;foo&gt; &amp;sp) //Again, no copy here  
    {    
        ...  
        sp-&gt;do_something();  
        ...  
    }  

I can&#39;t see in both cases a good reason to pass the `boost::shared_ptr&lt;foo&gt;` by value instead of by reference. Passing by value would only &quot;temporarily&quot; increment the reference count due to the copying, and then decrement it when exiting the function scope.
Am I overlooking something?

Just to clarify, after reading several answers: I perfectly agree on the premature-optimization concerns, and I always try to first-profile-then-work-on-the-hotspots. My question was more from a purely technical code-point-of-view, if you know what I mean.

C++ - passing references to std::shared_ptr or boost::shared_ptr

I am in the process of writing a text editor.  After looking at other text editors I have noticed that a number of them refer to a &quot;soft&quot; versus &quot;hard&quot; wrap.  What is the difference?  I can&#39;t seem to find the answer by searching.

Difference between hard wrap and soft wrap?

I&#39;m wondering if it&#39;s possible to define a foreign key in a models.py file in Django that is a reference to a table in another app?

In other words, I have two apps, called cf and profiles, and in cf/models.py I have (amongst other things):

    class Movie(models.Model):
        title = models.CharField(max_length=255)

and in profiles/models.py I want to have:

    class MovieProperty(models.Model):
        movie = models.ForeignKey(Movie)
        
But I can&#39;t get it to work. I&#39;ve tried:

        movie = models.ForeignKey(cf.Movie)

and I&#39;ve tried importing cf.Movie at the beginning of models.py, but I always get errors, such as:

    NameError: name &#39;User&#39; is not defined

Am I breaking the rules by trying to tie two apps together in this way, or have I just got the syntax wrong?

Foreign key from one app into another in Django

I'm wondering if it's possible to define a foreign key in a models.py file in Django that is a reference to a table in another app?
In other words, I have two apps, called cf and profiles, and in cf/models.py I have (amongst other things):
<pre><code class="hljs language-arduino">class Movie(models.Model):
 title = models.CharField(max_length=255)
</code></pre>
and in profiles/models.py I want to have:
<pre><code class="hljs language-arduino">class MovieProperty(models.Model):
 movie = models.ForeignKey(Movie)
 
</code></pre>
But I can't get it to work. I've tried:
<pre><code class="hljs language-ini"> movie = models.ForeignKey(cf.Movie)
</code></pre>
and I've tried importing cf.Movie at the beginning of models.py, but I always get errors, such as:
<pre><code class="hljs language-csharp">NameError: name 'User' is not defined
</code></pre>
Am I breaking the rules by trying to tie two apps together in this way, or have I just got the syntax wrong?

Is it possible to terminate a running thread without setting/checking any flags/semaphores/etc.?

Is there any way to kill a Thread?

I am currently running the following code based on Chapter 12.5 of the Python Cookbook:

    from xml.parsers import expat
    
    class Element(object):
        def __init__(self, name, attributes):
            self.name = name
            self.attributes = attributes
            self.cdata = &#39;&#39;
            self.children = []
        def addChild(self, element):
            self.children.append(element)
        def getAttribute(self,key):
            return self.attributes.get(key)
        def getData(self):
            return self.cdata
        def getElements(self, name=&#39;&#39;):
            if name:
                return [c for c in self.children if c.name == name]
            else:
                return list(self.children)
    
    class Xml2Obj(object):
        def __init__(self):
            self.root = None
            self.nodeStack = []
        def StartElement(self, name, attributes):
            element = Element(name.encode(), attributes)
            if self.nodeStack:
                parent = self.nodeStack[-1]
                parent.addChild(element)
            else:
                self.root = element
            self.nodeStack.append(element)
        def EndElement(self, name):
            self.nodeStack.pop()
        def CharacterData(self,data):
            if data.strip():
                data = data.encode()
                element = self.nodeStack[-1]
                element.cdata += data
        def Parse(self, filename):
            Parser = expat.ParserCreate()
            Parser.StartElementHandler = self.StartElement
            Parser.EndElementHandler = self.EndElement
            Parser.CharacterDataHandler = self.CharacterData
            ParserStatus = Parser.Parse(open(filename).read(),1)
            return self.root

I am working with XML documents of about 1 GB in size. Does anyone know a faster way to parse these?



What is the fastest way to parse large XML docs in Python?

I&#39;ve a python script that has to launch a shell command for every file in a dir:

    import os
    
    files = os.listdir(&quot;.&quot;)
    for f in files:
        os.execlp(&quot;myscript&quot;, &quot;myscript&quot;, f)

This works fine for the first file, but after the &quot;myscript&quot; command has ended, the execution stops and does not come back to the python script.

How can I do? Do I have to `fork()` before `calling os.execlp()`?

Launch a shell command with in a python script, wait for the termination and return to the script

I&#39;ve been wondering this for some time. As the title say, which is faster, the actual function or simply raising to the half power?

&lt;b&gt;UPDATE&lt;/b&gt;

This is not a matter of premature optimization. This is simply a question of how the underlying code actually works. What is the theory of how Python code works?

I sent Guido van Rossum an email cause I really wanted to know the differences in these methods.

###My email:

&gt; There are at least 3 ways to do a square root in Python: math.sqrt, the
&gt; &#39;**&#39; operator and pow(x,.5).  I&#39;m just curious as to the differences in
&gt; the implementation of each of these.  When it comes to efficiency which
&gt; is better?

###His response:

&gt; pow and ** are equivalent; math.sqrt doesn&#39;t work for complex numbers,
&gt; and links to the C &lt;math.h&gt; sqrt() function. As to which one is
&gt; faster, I have no idea...





Which is faster in Python: x**.5 or math.sqrt(x)?

Does anyone know how the built in dictionary type for python is implemented?  My understanding is that it is some sort of hash table, but I haven&#39;t been able to find any sort of definitive answer.

How are Python&#39;s Built In Dictionaries Implemented?

In a Django form, how do I make a field read-only (or disabled)?

When the form is being used to create a new entry, all fields should be enabled - but when the record is in update mode some fields need to be read-only.

For example, when creating a new `Item` model, all fields must be editable, but while updating the record, is there a way to disable the `sku` field so that it is visible, but cannot be edited?

    class Item(models.Model):
        sku = models.CharField(max_length=50)
        description = models.CharField(max_length=200)
        added_by = models.ForeignKey(User)


    class ItemForm(ModelForm):
        class Meta:
            model = Item
            exclude = (&#39;added_by&#39;)

    def new_item_view(request):
        if request.method == &#39;POST&#39;:
            form = ItemForm(request.POST)
            # Validate and save
        else:
                form = ItemForm()
        # Render the view

Can class `ItemForm` be reused? What changes would be required in the `ItemForm` or `Item` model class? Would I need to write another class, &quot;`ItemUpdateForm`&quot;, for updating the item?

    def update_item_view(request):
        if request.method == &#39;POST&#39;:
            form = ItemUpdateForm(request.POST)
            # Validate and save
        else:
            form = ItemUpdateForm()


In a Django form, how do I make a field readonly (or disabled) so that it cannot be edited?

I have a model that looks like this:

    class Category(models.Model):
        name = models.CharField(max_length=60)
	
    class Item(models.Model):
        name = models.CharField(max_length=60)
        category = models.ForeignKey(Category)
	

I want select count (just the count) of items for each category, so in SQL it would be as simple as this:

    select category_id, count(id) from item group by category_id

Is there an equivalent of doing this &quot;the Django way&quot;? Or is plain SQL the only option? I am familiar with the &lt;em&gt;count( )&lt;/em&gt; method in Django, however I don&#39;t see how &lt;em&gt;group by&lt;/em&gt; would fit there.



Django equivalent for count and group by

I&#39;ve just done my first little webapp in django and I love it. I&#39;m about to start on converting an old production PHP site into django and as part its template, there is a navigation bar.

In PHP, I check each nav option&#39;s URL against the current URL, in the template code and apply a CSS class if they line up. It&#39;s horrendously messy.

Is there something better for django or a good way of handling the code in the template?

To start, how would I go about getting the current URL?

Navigation in django

I&#39;m trying to format numbers. Examples:

    1     =&gt; 1
    12    =&gt; 12
    123   =&gt; 123
    1234  =&gt; 1,234
    12345 =&gt; 12,345

It strikes as a fairly common thing to do but I can&#39;t figure out which filter I&#39;m supposed to use.

Edit: If you&#39;ve a generic Python way to do this, I&#39;m happy adding a formatted field in my model.

Format numbers in django templates

If I have a Django form such as:

    class ContactForm(forms.Form):
        subject = forms.CharField(max_length=100)
        message = forms.CharField()
        sender = forms.EmailField()

And I call the as_table() method of an instance of this form, Django will render the fields as the same order as specified above.

My question is how does Django know the order that class variables where defined?  

(Also how do I override this order, for example when I want to add a field from the classe&#39;s __init__ method?)

How does Django Know the Order to Render Form Fields?

I need to store some data in a Django model. These data are not equal to all instances of the model.

At first I thought about subclassing the model, but I’m trying to keep the application flexible. If I use subclasses, I’ll need to create a whole class each time I need a new kind of object, and that’s no good. I’ll also end up with a lot of subclasses only to store a pair of extra fields.

I really feel that a dictionary would be the best approach, but there’s nothing in the Django documentation about storing a dictionary in a Django model (or I can’t find it).

Any clues?

How to store a dictionary on a Django Model?

When I read Django code I often see in models what is called a &quot;slug&quot;. I am not quite sure what this is, but I do know it has something to do with URLs. How and when is this slug-thing supposed to be used? 

(I have read its definition in [this glossary][1].)

  [1]: http://docs.djangoproject.com/en/dev/glossary/



What is a &quot;slug&quot; in Django?

I have model Foo which has field bar. The bar field should be unique, but allow nulls in it, meaning I want to allow more than one record if bar field is `null`, but if it is not `null` the values must be unique.

Here is my model:

    class Foo(models.Model):
        name = models.CharField(max_length=40)
        bar = models.CharField(max_length=40, unique=True, blank=True, null=True, default=None)

And here is the corresponding SQL for the table:

    CREATE TABLE appl_foo
    (
        id serial NOT NULL,
         &quot;name&quot; character varying(40) NOT NULL,
        bar character varying(40),
        CONSTRAINT appl_foo_pkey PRIMARY KEY (id),
        CONSTRAINT appl_foo_bar_key UNIQUE (bar)
    )	

When using admin interface to create more than 1 foo objects where bar is null it gives me an error: &quot;Foo with this Bar already exists.&quot;

However when I insert into database (PostgreSQL):

    insert into appl_foo (&quot;name&quot;, bar) values (&#39;test1&#39;, null)
    insert into appl_foo (&quot;name&quot;, bar) values (&#39;test2&#39;, null)

This works, just fine, it allows me to insert more than 1 record with bar being null, so the database allows me to do what I want, it&#39;s just something wrong with the Django model. Any ideas?

&lt;b&gt;EDIT&lt;/b&gt;

The portability of the solution as far as DB is not an issue, we are happy with Postgres. 
I&#39;ve tried setting unique to a callable, which was my function returning True/False for specific values of _bar_, it didn&#39;t give any errors, however seamed like it had no effect at all.

So far, I&#39;ve removed the unique specifier from the _bar_ property and handling the _bar_ uniqueness in the application, however still looking for a more elegant solution. Any recommendations?


Unique fields that allow nulls in Django

I have a Django app that requires a `settings` attribute in the form of:

    RELATED_MODELS = (&#39;appname1.modelname1.attribute1&#39;,
                      &#39;appname1.modelname2.attribute2&#39;, 
                      &#39;appname2.modelname3.attribute3&#39;, ...)

Then hooks their post_save signal to update some other fixed model depending on the `attributeN` defined.

I would like to test this behaviour and tests should work even if this app is the only one in the project (except for its own dependencies, no other wrapper app need to be installed). How can I create and attach/register/activate mock models just for the test database? (or is it possible at all?)

Solutions that allow me to use test fixtures would be great.


Django: How to create a model dynamically just for testing

Okay, how would I do this?

    class Example(models.Model):
      parent_example = models.ForeignKey(Example)

I want to have a model have a foreign key reference to itself.  When I try to create this I get a django validation error that Example is not yet defined.


Content Type	Original Author	Original Content on Stackoverflow
Question	Ben	View Question on Stackoverflow
Solution 1 - Python	Michael Warkentin	View Answer on Stackoverflow
Solution 2 - Python	andorov	View Answer on Stackoverflow
Solution 3 - Python	Ben	View Answer on Stackoverflow

Foreign key from one app into another in Django

Python Problem Overview

Python Solutions

Solution 1 - Python

Solution 2 - Python

Solution 3 - Python

Difference between hard wrap and soft wrap?

C++ - passing references to std::shared_ptr or boost::shared_ptr

Attributions