<html><head><meta name="color-scheme" content="light dark"></head><body><pre style="word-wrap: break-word; white-space: pre-wrap;">\documentclass[14pt,compress,aspectratio=169]{beamer}

\input{macros.tex}

\title[OOP: Class Attributes]{Advanced Python}
\subtitle{Object Oriented Programming: class attributes}

\author[FOSSEE] {The FOSSEE Group}

\institute[IIT Bombay] {Department of Aerospace Engineering\\IIT Bombay}
\date[] {Mumbai, India}

\begin{document}

\begin{frame}
  \titlepage
\end{frame}

\begin{frame}
  \frametitle{Recap}
  \begin{itemize}
  \item Classes for encapsulation
  \item Inheritance
  \item \py{super,isinstance,issubclass}
  \item Containership
    \vspace*{0.3in}
  \item Look at class attributes/methods
  \end{itemize}
\end{frame}


\begin{frame}[fragile]
  \frametitle{Class/instance attributes}
  \begin{itemize}
  \item So far: instance attributes
  \end{itemize}
\begin{lstlisting}
class A:
    x = 1

In []: a = A()
In []: a.x
Out[]: 1

In []: b = A()
In []: assert b.x == a.x
\end{lstlisting}
\end{frame}

\begin{frame}[fragile]
  \frametitle{Class/instance attributes}
\begin{lstlisting}
class A:
    x = 1

In []: a, b = A(), A()

In []: A.x = 2
In []: b.x
Out[]: 2
\end{lstlisting}
\end{frame}

\begin{frame}[fragile]
  \frametitle{Class/instance attributes: beware}
\begin{lstlisting}
class A:
    x = 1

In []: a = A()
In []: a.x = 1
In []: A.x = 2
In []: a.x
Out[]: ??
\end{lstlisting}
\end{frame}

\begin{frame}
  \frametitle{Understanding the scope}
  \begin{itemize}
  \item If \py{a.x} is not an instance attribute
  \item Python looks in the class for the attribute!
    \pause
    \vspace*{0.25in}
  \item So \py{a.x = 1} sets an instance attribute!
  \end{itemize}
\end{frame}

\begin{frame}[fragile,fragile]
  \frametitle{Class attribute scope}
\begin{lstlisting}
class A:
    x = 1
    def f(self):
        return self.x + 1

In []: a = A()
In []: a.f()
\end{lstlisting}
  \pause
\begin{lstlisting}
Out[]: 2
\end{lstlisting}
\end{frame}


\begin{frame}[fragile]
  \frametitle{Class methods}
  \begin{itemize}
  \item Can we have methods on the class?
  \end{itemize}
\begin{lstlisting}
class A:
    @classmethod
    def method(cls, x):
        print(cls, x)

In []: A.method(1)
&lt;class '__main__.A'&gt; 1
\end{lstlisting}
\end{frame}

\begin{frame}
  \frametitle{Class methods}
  \begin{itemize}
  \item \py{@classmethod} is a special decorator
  \item Makes the method a class method
  \item Implicit argument is the class (and not the instance)
  \item Useful when you don't want to construct the object
  \end{itemize}
\end{frame}

\begin{frame}
  \frametitle{Summary}
  \begin{itemize}
  \item Class attributes
  \item Class methods and the \py{@classmethod} decorator
  \end{itemize}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{frame}[plain, fragile]
  \frametitle{Exercise: Point class attribute}
  \begin{block}{}
    Create a simple \py{Point} class with class attributes \py{x, y} that
    default to 0.0.
  \end{block}

\begin{lstlisting}
In []: p = Point()
In []: p.x, p.y
Out[]: (0.0, 0.0)
\end{lstlisting}
\end{frame}


\begin{frame}[plain, fragile]
  \frametitle{Solution}
\begin{lstlisting}
class Point:
    x = 0.0
    y = 0.0
\end{lstlisting}
\end{frame}

\begin{frame}[plain, fragile]
  \frametitle{Exercise: classmethod}
  \begin{block}{}
    Create a simple \py{Point} class with instance attributes \py{x, y} but
    add a class method \py{polar(r, theta)} that takes coordinates in $(r,
    \theta)$ form but returns a suitable point. Remember that, $x=r
    \cos(\theta), y=r\sin(\theta)$. For example:
  \end{block}

\begin{lstlisting}
In []: p = Point.polar(1.0, 0.0)
In []: p.x, p.y
Out[]: (1.0, 0.0)
\end{lstlisting}

\end{frame}


\begin{frame}[plain, fragile]
  \frametitle{Solution}
\begin{lstlisting}
from math import sin, cos
class Point:
    def __init__(self, x=0.0, y=0.0):
        self.x = x
        self.y = y
    @classmethod
    def polar(cls, r, theta):
        x = r*cos(theta)
        y = r*sin(theta)
        return cls(x, y)

\end{lstlisting}
\end{frame}



\end{document}
</pre></body></html>