8. Objects¶

8.1. Object-Oriented Programming¶

Why object-oriented programming?

import jupyter_manim
from manimlib.imports import *

%%manim HelloWorld -l
class HelloWorld(Scene):
    def construct(self):
        self.play(Write(TextMobject('Hello, World!')))

HelloWorld is a specific Scene that is
constructed by playing an animation that Write
the TextMobject of the message 'Hello, World!'.

Exercise Try changing

Mobjects: TextMobject('Hello, World!') to TexMobject(r'E=mc^2') or Circle() or Square().
Animation objects: Write to FadeIn or GrowFromCenter.

See the documentation for other choices.

More complicated behavior can be achieved by using different objects.

%%html
<iframe width="912" height="513" src="https://www.youtube.com/embed/ENMyFGmq5OA" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

What is an object?

Almost everything is an object in Python.

isinstance?
isinstance(1, object), isinstance(1.0, object), isinstance('1', object)

(True, True, True)

A function is also a first-class object object.

isinstance(print, object), isinstance(''.isdigit, object)

(True, True)

A data type is also an object.

# chicken and egg relationship
isinstance(type, object), isinstance(object, type), isinstance(object, object)

(True, True, True)

Python is a class-based object-oriented programming language:

Each object is an instance of a class (also called type in Python).
An object is a collection of members/attributes, each of which is an object.

hasattr?
hasattr(str, 'isdigit')

True

Different objects of a class

have the same set of attributes as that of the class, but
the attribute values can be different.

dir?
dir(1)==dir(int), complex(1, 2).imag != complex(1, 1).imag

(True, True)

How to operate on an object?

A class can define a function as an attribute for all its instances.
Such a function is called a method or member function.

complex.conjugate(complex(1, 2)), type(complex.conjugate)

((1-2j), method_descriptor)

A method can be accessed by objects of the class:

complex(1, 2).conjugate(), type(complex(1, 2).conjugate)

((1-2j), builtin_function_or_method)

complex(1,2).conjugate is a callable object:

Its attribute __self__ is assigned to complex(1,2).
When called, it passes __self__ as the first argument to complex.conjugate.

callable(complex(1,2).conjugate), complex(1,2).conjugate.__self__

(True, (1+2j))

8.2. File Objects¶

How to read a text file?

Consider reading a csv (comma separated value) file:

!more 'contact.csv'

name, email, phone
Amelia Hawkins,dugorre@lufu.cg,(414) 524-6465
Alta Perez,bos@fiur.sc,(385) 247-9001
Tai Ming Chan,tmchan@cityu.edu.hk,(634) 234-7294
Annie Zimmerman,okodag@saswuf.mn,(259) 862-1082
Eula Crawford,ve@rorohte.mx,(635) 827-9819
Clayton Atkins,vape@nig.eh,(762) 271-7090
Hallie Day,kozzazazi@ozakewje.am,(872) 949-5878
Lida Matthews,joobu@pabnesis.kg,(213) 486-8330
Amelia Pittman,nulif@uposzag.au,(800) 303-3234

To read the file by a Python program:

f = open('contact.csv')  # create a file object for reading
print(f.read())   # return the entire content
f.close()         # close the file

name, email, phone
Amelia Hawkins,dugorre@lufu.cg,(414) 524-6465
Alta Perez,bos@fiur.sc,(385) 247-9001
Tai Ming Chan,tmchan@cityu.edu.hk,(634) 234-7294
Annie Zimmerman,okodag@saswuf.mn,(259) 862-1082
Eula Crawford,ve@rorohte.mx,(635) 827-9819
Clayton Atkins,vape@nig.eh,(762) 271-7090
Hallie Day,kozzazazi@ozakewje.am,(872) 949-5878
Lida Matthews,joobu@pabnesis.kg,(213) 486-8330
Amelia Pittman,nulif@uposzag.au,(800) 303-3234

open is a function that creates a file object and assigns it to f.
Associated with the file object,

read returns the entire content of the file as a string.
close flushes and closes the file.

Why close a file?

If not, depending on the operating system,

other programs may not be able to access the file, and
changes may not be written to the file.

To ensure a file is closed properly, we can use the with statement:

with open('contact.csv') as f:
    print(f.read())

name, email, phone
Amelia Hawkins,dugorre@lufu.cg,(414) 524-6465
Alta Perez,bos@fiur.sc,(385) 247-9001
Tai Ming Chan,tmchan@cityu.edu.hk,(634) 234-7294
Annie Zimmerman,okodag@saswuf.mn,(259) 862-1082
Eula Crawford,ve@rorohte.mx,(635) 827-9819
Clayton Atkins,vape@nig.eh,(762) 271-7090
Hallie Day,kozzazazi@ozakewje.am,(872) 949-5878
Lida Matthews,joobu@pabnesis.kg,(213) 486-8330
Amelia Pittman,nulif@uposzag.au,(800) 303-3234

The with statement applies to any context manager that provides the methods

__enter__ for initialization, and
__exit__ for finalization.

with open('contact.csv') as f:
    print(f, hasattr(f, '__enter__'), hasattr(f, '__exit__'), sep='\n')

<_io.TextIOWrapper name='contact.csv' mode='r' encoding='UTF-8'>
True
True

f.__enter__ is called after the file object is successfully created and assigned to f, and
f.__exit__ is called at the end, which closes the file.
f.closed indicates whether the file is closed.

f.closed

True

We can iterate a file object in a for loop,
which implicitly call the method __iter__ to read a file line by line.

with open('contact.csv') as f:
    for line in f:
        print(line, end='')

hasattr(f, '__iter__')

name, email, phone
Amelia Hawkins,dugorre@lufu.cg,(414) 524-6465
Alta Perez,bos@fiur.sc,(385) 247-9001
Tai Ming Chan,tmchan@cityu.edu.hk,(634) 234-7294
Annie Zimmerman,okodag@saswuf.mn,(259) 862-1082
Eula Crawford,ve@rorohte.mx,(635) 827-9819
Clayton Atkins,vape@nig.eh,(762) 271-7090
Hallie Day,kozzazazi@ozakewje.am,(872) 949-5878
Lida Matthews,joobu@pabnesis.kg,(213) 486-8330
Amelia Pittman,nulif@uposzag.au,(800) 303-3234

True

Exercise Print only the first 5 lines of the file contact.csv.

with open('contact.csv') as f:
    ### BEGIN SOLUTION
    for i, line in enumerate(f):
        print(line, end='')
        if i >= 5: break
    ### END SOLUTION

name, email, phone
Amelia Hawkins,dugorre@lufu.cg,(414) 524-6465
Alta Perez,bos@fiur.sc,(385) 247-9001
Tai Ming Chan,tmchan@cityu.edu.hk,(634) 234-7294
Annie Zimmerman,okodag@saswuf.mn,(259) 862-1082
Eula Crawford,ve@rorohte.mx,(635) 827-9819

How to write to a text file?

Consider backing up contact.csv to a new file:

destination = 'private/new_contact.csv'

The directory has to be created first if it does not exist:

import os
os.makedirs(os.path.dirname(destination), exist_ok=True)

os.makedirs?
!ls

contact.csv  media  Objects.ipynb  private

To write to the destination file:

with open('contact.csv') as source_file:
    with open(destination, 'w') as destination_file:
        destination_file.write(source_file.read())

destination_file.write?
!more {destination}

name, email, phone
Amelia Hawkins,dugorre@lufu.cg,(414) 524-6465
Alta Perez,bos@fiur.sc,(385) 247-9001
Tai Ming Chan,tmchan@cityu.edu.hk,(634) 234-7294
Annie Zimmerman,okodag@saswuf.mn,(259) 862-1082
Eula Crawford,ve@rorohte.mx,(635) 827-9819
Clayton Atkins,vape@nig.eh,(762) 271-7090
Hallie Day,kozzazazi@ozakewje.am,(872) 949-5878
Lida Matthews,joobu@pabnesis.kg,(213) 486-8330
Amelia Pittman,nulif@uposzag.au,(800) 303-3234

The argument 'w' to open sets the file object to write mode.
The method write writes the input strings to the file.

Exercise We can also use a mode to append new content to a file.
Complete the following code to append new_data to the file destination.

new_data = 'Effie, Douglas,galnec@naowdu.tc, (888) 311-9512'
with open(destination, 'a') as f:
    ### BEGIN SOLUTION
    f.write('\n')
    f.write(new_data)
    ### END SOLUTION
!more {destination}

name, email, phone
Amelia Hawkins,dugorre@lufu.cg,(414) 524-6465
Alta Perez,bos@fiur.sc,(385) 247-9001
Tai Ming Chan,tmchan@cityu.edu.hk,(634) 234-7294
Annie Zimmerman,okodag@saswuf.mn,(259) 862-1082
Eula Crawford,ve@rorohte.mx,(635) 827-9819
Clayton Atkins,vape@nig.eh,(762) 271-7090
Hallie Day,kozzazazi@ozakewje.am,(872) 949-5878
Lida Matthews,joobu@pabnesis.kg,(213) 486-8330
Amelia Pittman,nulif@uposzag.au,(800) 303-3234
Effie, Douglas,galnec@naowdu.tc, (888) 311-9512

How to delete a file?

Note that the file object does not provide any method to delete the file.
Instead, we should use the function remove of the os module.

if os.path.exists(destination):
    os.remove(destination)

8.3. String Objects¶

How to search for a substring in a string?

A string object has the method find to search for a substring.
E.g., to find the contact information of Tai Ming:

str.find?
with open('contact.csv') as f:
    for line in f:
        if line.find('Tai Ming') != -1:
            record = line
            print(record)
            break

Tai Ming Chan,tmchan@cityu.edu.hk,(634) 234-7294

How to split and join strings?

A string can be split according to a delimiter using the split method.

record.split(',')

['Tai Ming Chan', 'tmchan@cityu.edu.hk', '(634) 234-7294\n']

The list of substrings can be joined back together using the join methods.

print('\n'.join(record.split(',')))

Tai Ming Chan
tmchan@cityu.edu.hk
(634) 234-7294

Exercise Print only the phone number (last item) in record. Use the method rstrip or strip to remove unnecessary white spaces at the end.

str.rstrip?
### BEGIN SOLUTION
print(record.split(',')[-1].rstrip())
### END SOLUTION

(634) 234-7294

Exercise Print only the name (first item) in record but with

surname printed first with all letters in upper case
followed by a comma, a space, and
the first name as it is in record.

E.g., Tai Ming Chan should be printed as CHAN, Tai Ming.

Hint: Use the methods upper and rsplit (with the parameter maxsplit=1).

str.rsplit?
### BEGIN SOLUTION
first, last = record.split(',')[0].rsplit(' ', maxsplit=1)
print('{}, {}'.format(last.upper(),first))
### END SOLUTION

CHAN, Tai Ming

8.4. Operator Overloading¶

8.4.1. What is overloading?¶

Recall that the addition operation + behaves differently for different types.

for x, y in (1, 1), ('1', '1'), (1, '1'):
    print(f'{x!r:^5} + {y!r:^5} = {x+y!r}')

  1   +   1   = 2
 '1'  +  '1'  = '11'

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-32-297be71279db> in <module>
      1 for x, y in (1, 1), ('1', '1'), (1, '1'):
----> 2     print(f'{x!r:^5} + {y!r:^5} = {x+y!r}')

TypeError: unsupported operand type(s) for +: 'int' and 'str'

Having an operator perform differently based on its argument types is called operator overloading.
+ is called a generic operator.
We can also have function overloading to create generic functions.

8.4.2. How to dispatch on type?¶

The strategy of checking the type for the appropriate implementation is called dispatching on type.

A naive idea is to put all different implementations together with case-by-case checks of operand types.

def add_case_by_case(x, y):
    if isinstance(x, int) and isinstance(y, int):
        print('Do integer summation...')
    elif isinstance(x, str) and isinstance(y, str):
        print('Do string concatenation...')
    else:
        print('Return a TypeError...')
    return x + y  # replaced by internal implementations


for x, y in (1, 1), ('1', '1'), (1, '1'):
    print(f'{x!r:^10} + {y!r:^10} = {add_case_by_case(x,y)!r}')

Do integer summation...
    1      +     1      = 2
Do string concatenation...
   '1'     +    '1'     = '11'
Return a TypeError...

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-33-85c671bf17e3> in <module>
     10 
     11 for x, y in (1, 1), ('1', '1'), (1, '1'):
---> 12     print(f'{x!r:^10} + {y!r:^10} = {add_case_by_case(x,y)!r}')

<ipython-input-33-85c671bf17e3> in add_case_by_case(x, y)
      6     else:
      7         print('Return a TypeError...')
----> 8     return x + y  # replaced by internal implementations
      9 
     10 

TypeError: unsupported operand type(s) for +: 'int' and 'str'

It can get quite messy with all possible types and combinations.

for x, y in ((1, 1.1), (1, complex(1, 2)), ((1, 2), (1, 2))):
    print(f'{x!r:^10} + {y!r:^10} = {x+y!r}')

    1      +    1.1     = 2.1
    1      +   (1+2j)   = (2+2j)
  (1, 2)   +   (1, 2)   = (1, 2, 1, 2)

What about new data types?

from fractions import Fraction  # non-built-in type for fractions
for x, y in ((Fraction(1, 2), 1), (1, Fraction(1, 2))):
    print(f'{x} + {y} = {x+y}')

1/2 + 1 = 3/2
1 + 1/2 = 3/2

Weaknesses of the naive approach:

New data types require rewriting the addition operation.
A programmer may not know all other types and combinations to rewrite the code properly.

8.4.3. How to have data-directed programming?¶

The idea is to treat an implementation as a datum that can be returned by the operand types.

x + y is a syntactic sugar that
invokes the method type(x).__add__(x,y) of type(x) to do the addition.

for x, y in (Fraction(1, 2), 1), (1, Fraction(1, 2)):
    print(f'{x} + {y} = {type(x).__add__(x,y)}')  # instead of x + y

1/2 + 1 = 3/2
1 + 1/2 = NotImplemented

The first case calls Fraction.__add__, which provides a way to add int to Fraction.
The second case calls int.__add__, which cannot provide any way of adding Fraction to int. (Why not?)

Why return a NotImplemented object instead of raising an error/exception?

This allows + to continue to handle the addition by
dispatching on Fraction to call its reverse addition method __radd__.

%%mytutor -h 500
from fractions import Fraction
def add(x, y):
    '''Simulate the + operator.'''
    sum = x.__add__(y)
    if sum is NotImplemented:
        sum = y.__radd__(x)
    return sum


for x, y in (Fraction(1, 2), 1), (1, Fraction(1, 2)):
    print(f'{x} + {y} = {add(x,y)}')

The object-oriented programming techniques involved are formally called:

Polymorphism: Different types can have different implementations of the __add__ method.
Single dispatch: The implementation is chosen based on one single type at a time.

Remarks:

A method with starting and trailing double underscores in its name is called a dunder method.
Dunder methods are not intended to be called directly. E.g., we normally use + instead of __add__.
Other operators have their corresponding dunder methods that overloads the operator.

8.5. Object Aliasing¶

When are two objects identical?

Two objects are the same if they occupy the same memory.
The keyword is checks whether two objects are the same object.
The function id returns a unique id number for each object.

%%mytutor -h 400
x, y = complex(1,2), complex(1,2)
z = x

for expr in 'id(x)', 'id(y)', 'id(z)', 'x == y == z', 'x is y', 'x is z':
    print(expr,eval(expr))

As the box-pointer diagram shows:

x is not y because they point to objects at different memory locations,
even though the objects have the same type and value.
x is z because the assignment z = x binds z to the same memory location x points to.
z is said to be an alias (another name) of x.

Should we use is or ==?

is is faster but:

1 is 1, 1 is 1., 1 == 1.

(True, False, True)

1 is 1. returns false because 1 is int but 1. is float.
== calls the method __eq__ of float which returns mathematical equivalence.

Can we use is for integer comparison?

x, y = 1234, 1234
1234 is 1234, x is y

(True, False)

No. The behavior of is is not entirely predictable.

When should we use is?

is can be used for built-in constants such as None and NotImplemented
because there can only be one instance of each of them.

CS1302