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+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%Tutorial slides on Python.
+%
+% Author: FOSSEE
+% Copyright (c) 2017, FOSSEE, IIT Bombay
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+\documentclass[14pt,compress]{beamer}
+\input{macros.tex}
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+% Title page
+\title[Functions]{Python language: Functions}
+
+\author[FOSSEE Team] {The FOSSEE Group}
+
+\institute[IIT Bombay] {Department of Aerospace Engineering\\IIT Bombay}
+\date[] {Mumbai, India}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+% DOCUMENT STARTS
+\begin{document}
+
+\begin{frame}
+  \titlepage
+\end{frame}
+
+\section{Functions}
+
+\begin{frame}[fragile]
+  \frametitle{Functions for abstraction}
+  \begin{itemize}
+  \item Reduce duplication of code
+  \item Fewer lines of code: lesser scope for bugs
+  \item Re-usability of code
+  \item Use functions written by others, without exactly knowing how
+    they do, what they are doing
+  \item \alert{Enter Functions!}
+  \end{itemize}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Defining functions}
+  \begin{itemize}
+  \item Consider the function \texttt{f(x) = x\textasciicircum{}2}
+  \item In Python:
+  \end{itemize}
+  \begin{lstlisting}
+   In []: def f(x):
+   .....:     return x*x
+   .....:
+   In []: f(1)
+   In []: f(1+2j)
+  \end{lstlisting}
+  \begin{itemize}
+  \item \typ{def} is a keyword
+  \item \typ{f} is the name of the function
+  \item \typ{x} the parameter of the function
+  \item \typ{return} is a keyword
+  \end{itemize}
+\end{frame}
+
+
+\begin{frame}[fragile]
+  \frametitle{Defining functions \ldots}
+  \begin{lstlisting}
+  In []: def greet():
+  .....:     print("Hello World!")
+  .....:
+
+  In []: greet()
+  \end{lstlisting}
+  \begin{itemize}
+  \item \texttt{greet} is a function that takes no arguments
+  \item It returns nothing explicitly
+  \item Implicitly, Python returns \typ{None}
+  \item \typ{None} is also a built-in, immutable data type
+  \end{itemize}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Defining functions \ldots}
+  \begin{lstlisting}
+  In []: def avg(a, b):
+  .....:     return (a + b)/2
+  .....:
+
+  In []: avg(12, 10)
+\end{lstlisting}
+\begin{itemize}
+\item \typ{avg} takes two arguments
+\item Returns one value
+\end{itemize}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Doc-strings}
+  \begin{itemize}
+  \item It's highly recommended that all functions have documentation
+  \item We write a doc-string along with the function definition
+  \end{itemize}
+  \begin{lstlisting}
+def avg(a, b):
+    """Returns the average of two
+    given numbers."""
+   return (a + b)/2
+
+In[]: avg?
+In[]: greet?
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Returning multiple values}
+  \begin{itemize}
+  \item Return area and perimeter of circle, given radius
+  \item Function needs to return two values
+  \end{itemize}
+  \begin{lstlisting}
+def circle(r):
+    """Returns area and perimeter of
+    circle given radius r"""
+    pi = 3.14
+    area = pi * r * r
+    perimeter = 2 * pi * r
+    return area, perimeter
+
+In []: circle(4)
+In []: a, p = circle(6)
+  \end{lstlisting}
+\end{frame}
+
+
+\subsection{Default \& Keyword Arguments}
+
+\begin{frame}[fragile]
+  \frametitle{Default arguments}
+  \begin{lstlisting}
+In []: round(2.484)
+In []: round(2.484, 2)
+
+In []: s.split() # split on spaces
+In []: s.split(';') # split on ';'
+
+In []: range(10)
+In []: range(1, 10)
+In []: range(1, 10, 2)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Default arguments \ldots}
+  \begin{lstlisting}
+In []: def welcome(greet, name="World"):
+.....:     print(greet, name)
+.....:
+
+In []: welcome("Hi", "Guido")
+In []: welcome("Hello")
+  \end{lstlisting}
+\end{frame}
+\begin{frame}[fragile]
+  \frametitle{Default arguments \ldots}
+  \begin{itemize}
+  \item Arguments with default values, should be placed at the end
+  \item The following definition is \alert{WRONG}
+  \end{itemize}
+  \begin{lstlisting}
+In []: def welcome(name="World", greet):
+.....:     print(greet, name)
+.....:
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile,plain]
+  \frametitle{Keyword Arguments}
+  \begin{lstlisting}
+In []: def welcome(greet, name="World"):
+.....:     print(greet, name)
+.....:
+
+In []: welcome("Hello", "James")
+
+In []: welcome("Hi", name="Guido")
+
+In []: welcome(name="Guido",greet="Hey")
+
+In []: welcome(name="Guido", "Hey")
+\end{lstlisting}
+Cannot have non-keyword args after kwargs
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Built-in functions}
+  \begin{itemize}
+  \item Variety of built-in functions are available
+  \item \typ{abs, any, all, len, max, min}
+  \item \typ{pow, range, sum, type}
+  \item Refer here:
+    \url{http://docs.python.org/library/functions.html}
+  \end{itemize}
+\end{frame}
+
+\subsection{Variable Scope}
+
+\begin{frame}[fragile]
+  \frametitle{Arguments are local}
+  \begin{lstlisting}
+   In []: def change(q):
+   .....:     q = 10
+   .....:     print(q)
+   .....:
+
+   In []: change(1)
+   In []: print(q)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Variables inside function are local}
+  \begin{lstlisting}
+  In []: n = 5
+  In []: def change():
+  .....:     n = 10
+  .....:     print(n)
+  .....:
+  In []: change()
+  In []: print(n)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{If not defined, look in parent scope}
+  \begin{lstlisting}
+  In []: n = 5
+  In []: def scope():
+  .....:     print(n)
+  .....:
+  In []: scope()
+\end{lstlisting}
+\begin{itemize}
+\item Note that \typ{n} was not defined in \typ{scope}
+\item It looked for the variable in previous scope
+\item See: \url{pythontutor.com}
+\end{itemize}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{\texttt{global}}
+ \begin{itemize}
+  \item Use the \texttt{global} statement to assign to global variables
+  \end{itemize}
+  \begin{lstlisting}
+  In[]: def change():
+  ....:     global n
+  ....:     n = 10
+  ....:     print(n)
+  ....:
+  In[]: change()
+  In[]: print(n)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Mutable variables}
+  \begin{itemize}
+  \item Behavior is different when assigning to a list element/slice
+  \item Python looks up for the name, from innermost scope outwards,
+    until the name is found
+  \end{itemize}
+  \begin{lstlisting}
+In []: name = ['Mr.', 'Steve','Gosling']
+In []: def change_name():
+.....:     name[0] = 'Dr.'
+.....:
+In []: change_name()
+In []: print(name)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Passing Arguments \ldots}
+  \begin{lstlisting}
+In []: n = 5
+In []: def change(n):
+.....:     n = 10
+.....:     print(n, "inside change")
+.....:
+In []: change(n)
+In []: print(n)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Passing Arguments \ldots}
+\begin{lstlisting}
+In []: name = ['Mr.', 'Steve','Gosling']
+In []: def change_name(x):
+.....:     x[0] = 'Dr.'
+.....:     print(x, 'in change')
+.....:
+In []: change_name(name)
+In []: print(name)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Passing Arguments \ldots}
+\begin{lstlisting}
+In []: name = ['Mr.', 'Steve','Gosling']
+In []: def change_name(x):
+.....:     x = [1,2,3]
+.....:     print(x, 'in change')
+.....:
+In []: change_name(name)
+In []: print(name)
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Passing Arguments \ldots}
+\begin{lstlisting}
+In []: name = ['Mr.', 'Steve','Gosling']
+In []: def change_name(x):
+.....:     x[:] = [1,2,3]
+.....:     print(x, 'in change')
+.....:
+In []: change_name(name)
+In []: print(name)
+  \end{lstlisting}
+\end{frame}
+
+
+\subsection{Examples}
+
+\begin{frame}[fragile]
+  \frametitle{Functions: example}
+  \begin{lstlisting}
+def signum( r ):
+    """returns 0 if r is zero
+    -1 if r is negative
+    +1 if r is positive"""
+    if r < 0:
+        return -1
+    elif r > 0:
+        return 1
+    else:
+        return 0
+  \end{lstlisting}
+  \emphbar{Note docstrings}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle {What does this function do?}
+  \begin{lstlisting}
+def what(n):
+    if n < 0: n = -n
+    while n > 0:
+        if n % 2 == 1:
+            return False
+        n /= 10
+    return True
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{What does this function do?}
+\begin{lstlisting}
+def what(n):
+    i = 1
+    while i * i < n:
+        i += 1
+    return i * i == n, i
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle {What does this function do?}
+  \begin{lstlisting}
+def what(x, n):
+    if n < 0:
+       n = -n
+       x = 1.0 / x
+
+    z = 1.0
+    while n > 0:
+        if n % 2 == 1:
+            z *= x
+        x *= x
+        n //= 2
+    return z
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Nested functions}
+  \begin{lstlisting}
+    def f(x):
+        def g(x):
+            return x+1
+        return g(x)**2
+  \end{lstlisting}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitle{Nested functions: returning functions}
+  \begin{lstlisting}
+    def f():
+        def g(x):
+            return x+1
+        return g
+
+    In []: func = f()
+    In []: func(1)
+    In []: f()(1)  # Also valid!
+  \end{lstlisting}
+\end{frame}
+
+
+\begin{frame}
+  \frametitle{Summary}
+  \begin{itemize}
+  \item Defining functions
+  \item Taking either 0, or more arguments
+  \item Returning \typ{None} implicitly or any number of values
+  \item Default and keyword arguments
+  \item Variable scope, \typ{global}
+  \item Mutable and immutable arguments
+  \item Nested functions
+  \end{itemize}
+\end{frame}
+
+\end{document}