Lecture 3 Python Functions and Classes¶

For longer and more complex tasks, it is important to organize your code into reuseable elements. For example, if you find yourself cutting and pasting the same or similar lines of code over and over, you probably need to define a function to encapsulate that code and make it reusable. An important principle in programming in DRY: "don't repeat yourself". Repetition is tedious and opens you up to errors. Strive for elegance and simplicity in your programs.

Functions¶

Functions are a central part of advanced python programming. Functions take some inputs ("arguments") and do something in response. Usually functions return something, but not always.

In [48]:

# define a function
def say_hello():
    """Return the word hello."""
    return 'Hello'

# define a function def say_hello(): """Return the word hello.""" return 'Hello'

In [49]:

# functions are also objects
type(say_hello)

# functions are also objects type(say_hello)

Out[49]:

function

In [50]:

# this doesnt call
say_hello?

# this doesnt call say_hello?

Signature: say_hello()
Docstring: Return the word hello.
File:      /var/folders/7b/6t7qqfj57bb0_ml_y_5bw86r0000gn/T/ipykernel_76551/1650374671.py
Type:      function

In [51]:

# this does
say_hello()

# this does say_hello()

Out[51]:

'Hello'

In [52]:

# assign the result to something
res = say_hello()
res

# assign the result to something res = say_hello() res

Out[52]:

'Hello'

In [12]:

# take some arguments
def say_hello_to(name):
    """Return a greeting to `name`"""
    return 'Hello ' + name

# take some arguments def say_hello_to(name): """Return a greeting to `name`""" return 'Hello ' + name

In [13]:

# intended usage
say_hello_to('World')

# intended usage say_hello_to('World')

Out[13]:

'Hello World'

In [14]:

say_hello_to(10)

say_hello_to(10)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[14], line 1
----> 1 say_hello_to(10)

Cell In[12], line 4, in say_hello_to(name)
      2 def say_hello_to(name):
      3     """Return a greeting to `name`"""
----> 4     return 'Hello ' + name

TypeError: can only concatenate str (not "int") to str

In [15]:

# redefine the function
def say_hello_to(name):
    """Return a greeting to `name`"""
    return 'Hello ' + str(name)

# redefine the function def say_hello_to(name): """Return a greeting to `name`""" return 'Hello ' + str(name)

In [16]:

say_hello_to(10)

say_hello_to(10)

Out[16]:

'Hello 10'

In [17]:

# take an optional keyword argument
def say_hello_language(name, chinese=False):
    """Say hello in multiple languages."""
    if chinese:
        greeting = 'Ni Hao '
    else:
        greeting = 'Hello '
    return greeting + name

# take an optional keyword argument def say_hello_language(name, chinese=False): """Say hello in multiple languages.""" if chinese: greeting = 'Ni Hao ' else: greeting = 'Hello ' return greeting + name

In [18]:

print(say_hello_language('Matt'))
print(say_hello_language('Siyi', chinese=True))

print(say_hello_language('Matt')) print(say_hello_language('Siyi', chinese=True))

Hello Matt
Ni Hao Siyi

In [19]:

# flexible number of arguments
def say_hello_to_everyone(*args):
    return ['hello ' + str(a) for a in args]

# flexible number of arguments def say_hello_to_everyone(*args): return ['hello ' + str(a) for a in args]

In [20]:

say_hello_to_everyone('Matt', 'Siyi', 'Kerry')

say_hello_to_everyone('Matt', 'Siyi', 'Kerry')

Out[20]:

['hello Matt', 'hello Siyi', 'hello Kerry']

Pure vs. Impure Functions¶

Functions that don't modify their arguments or produce any other side-effects are called pure.

Functions that modify their arguments or cause other actions to occur are called impure.

Below is an impure function.

In [53]:

# The function doesn't return anything, but it changes the input arguments. 
def remove_last_from_list(input_list):
    input_list.pop()

# The function doesn't return anything, but it changes the input arguments. def remove_last_from_list(input_list): input_list.pop()

In [22]:

names = ['Matt', 'Siyi', 'Kerry']
remove_last_from_list(names)
print(names)
remove_last_from_list(names)
print(names)

names = ['Matt', 'Siyi', 'Kerry'] remove_last_from_list(names) print(names) remove_last_from_list(names) print(names)

['Matt', 'Siyi']
['Matt']

We can do something similar with a pure function.

In general, pure functions are safer and more reliable.

In [23]:

def remove_last_from_list_pure(input_list):
    new_list = input_list.copy()
    new_list.pop()
    return new_list

def remove_last_from_list_pure(input_list): new_list = input_list.copy() new_list.pop() return new_list

In [24]:

names = ['Matt', 'Siyi', 'Kerry']
new_names = remove_last_from_list_pure(names)
print(names)
print(new_names)

names = ['Matt', 'Siyi', 'Kerry'] new_names = remove_last_from_list_pure(names) print(names) print(new_names)

['Matt', 'Siyi', 'Kerry']
['Matt', 'Siyi']

We could spend the rest of the day talking about functions, but we have to move on.

Classes¶

We have worked with many different types of python objects so far: strings, lists, dictionaries, etc. These objects have different attributes and respond in different ways to the built-in functions (len, etc.)

Python is an object oriented programming language.

Almost everything in Python is an object, with its properties and methods.

How can we make our own, custom objects? Answer: by defining classes.

A class to represent a Student¶

In [25]:

# Create a class named Student with a name. 
class Student:
    name = 'Matt'

# Create a class named Student with a name. class Student: name = 'Matt'

In [26]:

print(Student.name)

print(Student.name)

Matt

The __init__() function¶

All classes have a function called __init__(), which is always executed when the class is being initiated.

In [27]:

class Student:
    
    def __init__(self, name):
        self.name = name

class Student: def __init__(self, name): self.name = name

In [28]:

s1 = Student('Matt')
s1

s1 = Student('Matt') s1

Out[28]:

<__main__.Student at 0x111dfa250>

Our class only has a single attribute so far:

In [29]:

s1.name

s1.name

Out[29]:

'Matt'

Let's add more, along with some input validation:

In [54]:

class Student:
    
    def __init__(self, name, age, major):
        self.name = name.upper()
        self.major = major
        self.age = age

class Student: def __init__(self, name, age, major): self.name = name.upper() self.major = major self.age = age

In [57]:

s1 = Student('Matt', 22, 'Environmental Science')

s1 = Student('Matt', 22, 'Environmental Science')

In [58]:

s1.major

s1.major

Out[58]:

'Environmental Science'

In [59]:

s1.name

s1.name

Out[59]:

'MATT'

In [60]:

s1.age

s1.age

Out[60]:

22

In [61]:

class Student:
    
    def __init__(self, name, age, major):
        self.name = name.upper()
        self.major = major
        
        if age<0:
            raise ValueError(f'Invalid age {age}')
        self.age = age

class Student: def __init__(self, name, age, major): self.name = name.upper() self.major = major if age<0: raise ValueError(f'Invalid age {age}') self.age = age

In [62]:

s1 = Student('Matt', -46, 'Environmental Science')
s1

s1 = Student('Matt', -46, 'Environmental Science') s1

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[62], line 1
----> 1 s1 = Student('Matt', -46, 'Environmental Science')
      2 s1

Cell In[61], line 8, in Student.__init__(self, name, age, major)
      5 self.major = major
      7 if age<0:
----> 8     raise ValueError(f'Invalid age {age}')
      9 self.age = age

ValueError: Invalid age -46

In [63]:

s1 = Student('Matt', 22, 'Environmental Science')
s1

s1 = Student('Matt', 22, 'Environmental Science') s1

Out[63]:

<__main__.Student at 0x1123fe6d0>

Now let's add a custom method:¶

In [36]:

class Student:
    
    def __init__(self, name, age, major):
        self.name = name.upper()
        self.major = major
        
        if age<0:
            raise ValueError(f'Invalid age {age}')
        self.age = age
    
    def is_ES(self):
        return self.major == 'Environmental Science'

class Student: def __init__(self, name, age, major): self.name = name.upper() self.major = major if age<0: raise ValueError(f'Invalid age {age}') self.age = age def is_ES(self): return self.major == 'Environmental Science'

In [37]:

s1 = Student('Matt', 22, 'Environmental Science')
s1.is_ES()

s1 = Student('Matt', 22, 'Environmental Science') s1.is_ES()

Out[37]:

True

In [38]:

s2 = Student('Siyi', 25, 'Ecology')
s2.is_ES()

s2 = Student('Siyi', 25, 'Ecology') s2.is_ES()

Out[38]:

False

Modify Object Properties¶

In [39]:

s1.age = 40

s1.age = 40

In [40]:

s1.age

s1.age

Out[40]:

40

In [41]:

del(s1.age)

del(s1.age)

In [42]:

s1.age

s1.age

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[42], line 1
----> 1 s1.age

AttributeError: 'Student' object has no attribute 'age'

In [43]:

s2.age

s2.age

Out[43]:

25

In [44]:

s1.is_ES()

s1.is_ES()

Out[44]:

True

In [ ]: