You need to use the [`__getitem__` method][1].

    class MyClass:
        def __getitem__(self, key):
            return key * 2
   
    myobj = MyClass()
    myobj[3] #Output: 6


And if you&#39;re going to be setting values you&#39;ll need to implement the [`__setitem__` method][2] too, otherwise this will happen:

    &gt;&gt;&gt; myobj[5] = 1
    Traceback (most recent call last):
      File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
    AttributeError: MyClass instance has no attribute &#39;__setitem__&#39;


  [1]: https://docs.python.org/reference/datamodel.html#object.__getitem__
  [2]: https://docs.python.org/reference/datamodel.html#object.__setitem__

To fully overload it you also need to implement the `__setitem__`and `__delitem__` methods.

**edit**

I almost forgot... if you want to completely emulate a list, you also need `__getslice__, __setslice__ and __delslice__`.

There are all documented in http://docs.python.org/reference/datamodel.html


You are looking for the `__getitem__` method. See http://docs.python.org/reference/datamodel.html, section 3.4.6

In MS SQL-Server, I can do:

   `SELECT ISNULL(Field,&#39;Empty&#39;) from Table`

But in PostgreSQL I get a syntax error. How do I emulate the `ISNULL()` functionality ?

What is the PostgreSQL equivalent for ISNULL()

What is the difference between varchar and varchar2?

What is the difference between varchar and varchar2 in Oracle?

What is the name of the method to override the `[]` operator (subscript notation) for a class in Python?

How to override the [] operator in Python?

<p>What is the name of the method to override the <code>[]</code> operator (subscript notation) for a class in Python?</p>


Is there a simple way to iterate over column name and value pairs?

My version of SQLAlchemy is 0.5.6

Here is the sample code where I tried using `dict(row)`:

    import sqlalchemy
    from sqlalchemy import *
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy.orm import sessionmaker
    
    print &quot;sqlalchemy version:&quot;,sqlalchemy.__version__ 
    
    engine = create_engine(&#39;sqlite:///:memory:&#39;, echo=False)
    metadata = MetaData()
    users_table = Table(&#39;users&#39;, metadata,
         Column(&#39;id&#39;, Integer, primary_key=True),
         Column(&#39;name&#39;, String),
    )
    metadata.create_all(engine) 
    
    class User(declarative_base()):
        __tablename__ = &#39;users&#39;
        
        id = Column(Integer, primary_key=True)
        name = Column(String)
        
        def __init__(self, name):
            self.name = name
    
    Session = sessionmaker(bind=engine)
    session = Session()
    
    user1 = User(&quot;anurag&quot;)
    session.add(user1)
    session.commit()
    
    # uncommenting next line throws exception &#39;TypeError: &#39;User&#39; object is not iterable&#39;
    #print dict(user1)
    # this one also throws &#39;TypeError: &#39;User&#39; object is not iterable&#39;
    for u in session.query(User).all():
        print dict(u)

Running this code on my system outputs:

    Traceback (most recent call last):
      File &quot;untitled-1.py&quot;, line 37, in &lt;module&gt;
        print dict(u)
    TypeError: &#39;User&#39; object is not iterable



How to convert SQLAlchemy row object to a Python dict?

I am trying to do this:

     {% for movie in movie_list | sort(movie.rating) %}

But that&#39;s not right...the documentation is vague...how do you do this in Jinja2?


How do you sort a list in Jinja2?

I have a `Decimal(&#39;3.9&#39;)` as part of an object, and wish to encode this to a JSON string which should look like `{&#39;x&#39;: 3.9}`. I don&#39;t care about precision on the client side, so a float is fine.

Is there a good way to serialize this? JSONDecoder doesn&#39;t accept Decimal objects, and converting to a float beforehand yields `{&#39;x&#39;: 3.8999999999999999}` which is wrong, and will be a big waste of bandwidth.


Python JSON serialize a Decimal object

I&#39;m using [Virtualenv][1] with profit on my development environment with `web.py`, `simplejson` and other web oriented packages.  
I&#39;m going to develop a simple python client using Qt to reuse some Api developed with web.py.  

Does anybody here had succesfully installed PyQt4 with Virtualenv?  
Is it possible?

I&#39;ve downloaded all the binaries and have `PyQt4` installed globally on my python2.6 directory.    
If I don&#39;t use `--no-site--packages` option, Virtualenv correctly includes `PyQt4` in my new sandbox but, obviously, with all the global packages that I don&#39;t need.
      
Is there a clean way to prepare a new sandbox with `--no-site--packages` option and then add `PyQt4` or `PySide` using `pip`, `easy_install` or some other magic trick?


  [1]: http://pypi.python.org/pypi/virtualenv

Is it possible to add PyQt4/PySide packages on a Virtualenv sandbox?

I am writing a simple Python program.  

My program seems to suffer from linear access to dictionaries, 
its run-time grows exponentially even though the algorithm is quadratic.  
I use a dictionary to memoize values. That seems to be a bottleneck.  

The values I&#39;m hashing are tuples of points.
Each point is: (x,y), 0 &lt;= x,y &lt;= 50  
Each key in the dictionary is: A tuple of 2-5 points: ((x1,y1),(x2,y2),(x3,y3),(x4,y4))

The keys are read many times more often than they are written.

Am I correct that python dicts suffer from linear access times with such inputs?  

As far as I know, sets have guaranteed logarithmic access times.  
How can I simulate dicts using sets(or something similar) in Python?  


*edit* As per request, here&#39;s a (simplified) version of the memoization function:

    def memoize(fun):
        memoized = {}
        def memo(*args):
            key = args
            if not key in memoized:
                memoized[key] = fun(*args)
            return memoized[key]
        return memo

Time complexity of accessing a Python dict

I&#39;m implementing vector class and I need to get an opposite of some vector. Is it possible to define this method using operator overloading?

Here&#39;s what I mean:

    Vector2f vector1 = -vector2;

Here&#39;s what I want this operator to accomplish:

    Vector2f&amp; oppositeVector(const Vector2f &amp;_vector)
    {
     x = -_vector.getX();
     y = -_vector.getY();
     
     return *this;
    }

Thanks.

How to overload unary minus operator in C++?

If I am creating my own class in Python, what function should I define so as to allow the use of the `in` operator, e.g.

    class MyClass(object):
        ...
    
    m = MyClass()
    
    if 54 in m:
        ...



Override Python&#39;s &#39;in&#39; operator?

I have a question about the return value of operator overloading in C++. Generally, I found two cases, one is return-by-value, and one is return-by-reference. So what&#39;s the underneath rule of that? Especially at the case when you can use the operator continuously, such as `cout&lt;&lt;x&lt;&lt;y`.

For example, when implementing a + operation &quot;string + (string)&quot;. how would you return the return value, by ref or by val.

return value of operator overloading in C++

There are two ways to overload operators for a C++ class:

### Inside class

    class Vector2
    {
    public:
        float x, y ;
        
        Vector2 operator+( const Vector2 &amp; other )
        {
            Vector2 ans ;
            ans.x = x + other.x ;
            ans.y = y + other.y ;
            return ans ;
        }
    } ;


### Outside class
    
    class Vector2
    {
    public:
        float x, y ;
    } ;
    
    Vector2 operator+( const Vector2&amp; v1, const Vector2&amp; v2 )
    {
        Vector2 ans ;
        ans.x = v1.x + v2.x ;
        ans.y = v1.y + v2.y ;
        return ans ;
    }


(Apparently in C# you can only use the &quot;outside class&quot; method.)

In C++, which way is more correct?  Which is preferable?

Operator overloading outside class

In C++, the concept of returning reference from the copy assignment operator is unclear to me. Why can&#39;t the copy assignment operator return a copy of the new object? In addition, if I have class `A`, and the following:

    A a1(param);
    A a2 = a1;
    A a3;
    
    a3 = a2; //&lt;--- this is the problematic line

The `operator=` is defined as follows:

    A A::operator=(const A&amp; a)
    {
    	if (this == &amp;a)
    	{
    		return *this;
    	}
    	param = a.param;
    	return *this;
    }

Content Type	Original Author	Original Content on Stackoverflow
Question	Sahas	View Question on Stackoverflow
Solution 1 - Python	Dave Webb	View Answer on Stackoverflow
Solution 2 - Python	Dave Kirby	View Answer on Stackoverflow
Solution 3 - Python	Confusion	View Answer on Stackoverflow

How to override the [] operator in Python?

Python Problem Overview

Python Solutions

Solution 1 - Python

Solution 2 - Python

Solution 3 - Python

What is the difference between varchar and varchar2 in Oracle?

What is the PostgreSQL equivalent for ISNULL()

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