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\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)
<class '__main__.A'> 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}
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