Python Tutorial

Python is a language which prides itself on simplicity of reading and understanding. If you'd like to read more about python go to https://en.wikipedia.org/wiki/Python_(programming_language) !

If you'd like to read further than this, or a more basic guide I would recommend: https://en.wikibooks.org/wiki/Non-Programmer%27s_Tutorial_for_Python_3

Here's an example python program to print something on the screen:

print('Hello, World!')
# This is a comment, it isn't run as code, but often they are helpful
Hello, World!

1. Arithmetic

Like every programming language, Python is a good calculator. Run the block of code below to make sure the answer is right!

1 + 1
2

The order of operations you learned in school applies, BEDMAS (brackets, exponents, division, multiplication, addition, subtraction):

8 + 6*2*3 - (15 - 13)
42

Numbers are valid Python code as are the common operators, +, /, * and -. You can write different types of numbers including integers, real numbers (floating point) and negative integers.

42 + 3.149 + -1
44.149

Since 42 is literally 42, we call these numbers literals. You are literally writing number in your Python code.

2. Variables

In python, variables are similar to C, Java and other languages - they store a certain value. Specifically, in python variables do not have a type. So, a string can also become an integer, and then even into a function! Literals are given types of string, integer, float, lists, objects, etc. But the variable's type can change.

x = 200.00
print("x =", x, "and is of type", type(x))
x = "Hello World!"
print("x =", x, "and is of type", type(x))

3. Exceptions in Python

Python only understands certain code. When you write something Python doesn't understand it throws an exception and tries to explain what went wrong, but it can only speak in a broken Pythonesque english. Let's see some examples by running these code blocks

variable_that_is_undefined
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-5-a887aa8d91a7> in <module>()
----> 1 variable_that_is_undefined

NameError: name 'variable_that_is_undefined' is not defined
print('Hello'
  File "<ipython-input-6-49c0f052bb79>", line 1
    print('Hello'
                 ^
SyntaxError: unexpected EOF while parsing
2000 / 0
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-7-d14d67f6c4a6> in <module>()
----> 1 2000 / 0

ZeroDivisionError: division by zero

Python tries to tell you where it stopped understanding, but in the above examples, each program is only 1 line long.

It also tries to show you where on the line the problem happened with caret ("^").

Finally it tells you the type of thing that went wrong, (NameError, SyntaxError, ZeroDivisionError) and a bit more information like "name 'gibberish' is not defined" or "unexpected EOF while parsing".

Unfortunately you might not find "unexpected EOF while parsing" too helpful. EOF stands for End of File, but what file? What is parsing? Python does it's best, but it does take a bit of time to develop a knack for what these messages mean. If you run into an error you don't understand please ask.

4. Strings

We have already seen a string literal in Python, "Hello, World!"

"Hello, World!"
'Hello, World!'

Text literals are surrounded by quotes. Without the quotes Hello by itself would be viewed as a variable name. You can use either double quotes (") or single quotes (') for text literals. Note that in python a character is just a string with 1 character and isn't a different data type.

Let's use strings:

print("Hello " * 5)
print("Hello" + "World")
Hello Hello Hello Hello Hello 
HelloWorld

Strings in Python are a bit more complicated because they have their own functions (operations to perform on them) which we can call to modify them:

print("Uppercase:", "Hello world".upper())
print("Lowercase:", "Hello world".lower())
print()

print("Strip all whitespace at end of string:", "Hello           ".strip(), "world")
print("Find location of 'world' in string:", "Hello world".find('world'))
print()

# C like string formatting where the "%s" is replaced by the values given
val = "Hello %s"
replaced_val = val % "world"
print("String formatting with %s like C:", replaced_val)

# We can have template strings where parts are specified on the fly
val = "Hello {name}"
replaced_val = val.format(name="world")
print("String formatting with .format():", replaced_val)
Uppercase: HELLO WORLD
Lowercase: hello world

Strip all whitespace at end of string: Hello world
Find location of 'world' in string: 6

String formatting with %s like C: Hello world
String formatting with .format(): Hello world

To convert another data type like float, integer, etc into a string, you can use the str() function:

str(1.0) + " cm"
'1.0 cm'

Indexed by Zero

For better or worse, everything in Python is index by 0 like C/C++ or Java. We will see this over and over again but for now if you call format like this:

"{0} like {1}".format("I", 'Python')
'I like Python'

We would call "I" the 0th string passed into the function .format() and 'Python' the 1st.

Multi line strings

Frequently you will have a string which spans multiple lines. There are 3 ways to handle this in python:

  • Use a backslash
  • Triple quotes
# Using a backslash is similar to C, where the compiler knows the line is being continued in the next line
print("this is \
a multiline string")

# In multiline strings, even newlines (\n) is added as a character. So, a newline gets printed.
print("""this is
a multiline string""")
this is a multiline string
this is
a multiline string

Exercise: Use the help() function

To find more about a specific enity in python, simply do help(<entity>). Try it out!

  • Find the help for the find() function we used earlier by typing help("abcd".find). How many arguments does it have ?
  • Find the help for the find() function and check what the function split() does ?
help(int)
Help on class int in module builtins:

class int(object)
 |  int(x=0) -> integer
 |  int(x, base=10) -> integer
 |  
 |  Convert a number or string to an integer, or return 0 if no arguments
 |  are given.  If x is a number, return x.__int__().  For floating point
 |  numbers, this truncates towards zero.
 |  
 |  If x is not a number or if base is given, then x must be a string,
 |  bytes, or bytearray instance representing an integer literal in the
 |  given base.  The literal can be preceded by '+' or '-' and be surrounded
 |  by whitespace.  The base defaults to 10.  Valid bases are 0 and 2-36.
 |  Base 0 means to interpret the base from the string as an integer literal.
 |  >>> int('0b100', base=0)
 |  4
 |  
 |  Methods defined here:
 |  
 |  __abs__(self, /)
 |      abs(self)
 |  
 |  __add__(self, value, /)
 |      Return self+value.
 |  
 |  __and__(self, value, /)
 |      Return self&value.
 |  
 |  __bool__(self, /)
 |      self != 0
 |  
 |  __ceil__(...)
 |      Ceiling of an Integral returns itself.
 |  
 |  __divmod__(self, value, /)
 |      Return divmod(self, value).
 |  
 |  __eq__(self, value, /)
 |      Return self==value.
 |  
 |  __float__(self, /)
 |      float(self)
 |  
 |  __floor__(...)
 |      Flooring an Integral returns itself.
 |  
 |  __floordiv__(self, value, /)
 |      Return self//value.
 |  
 |  __format__(...)
 |  
 |  __ge__(self, value, /)
 |      Return self>=value.
 |  
 |  __getattribute__(self, name, /)
 |      Return getattr(self, name).
 |  
 |  __getnewargs__(...)
 |  
 |  __gt__(self, value, /)
 |      Return self>value.
 |  
 |  __hash__(self, /)
 |      Return hash(self).
 |  
 |  __index__(self, /)
 |      Return self converted to an integer, if self is suitable for use as an index into a list.
 |  
 |  __int__(self, /)
 |      int(self)
 |  
 |  __invert__(self, /)
 |      ~self
 |  
 |  __le__(self, value, /)
 |      Return self<=value.
 |  
 |  __lshift__(self, value, /)
 |      Return self<<value.
 |  
 |  __lt__(self, value, /)
 |      Return self<value.
 |  
 |  __mod__(self, value, /)
 |      Return self%value.
 |  
 |  __mul__(self, value, /)
 |      Return self*value.
 |  
 |  __ne__(self, value, /)
 |      Return self!=value.
 |  
 |  __neg__(self, /)
 |      -self
 |  
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.
 |  
 |  __or__(self, value, /)
 |      Return self|value.
 |  
 |  __pos__(self, /)
 |      +self
 |  
 |  __pow__(self, value, mod=None, /)
 |      Return pow(self, value, mod).
 |  
 |  __radd__(self, value, /)
 |      Return value+self.
 |  
 |  __rand__(self, value, /)
 |      Return value&self.
 |  
 |  __rdivmod__(self, value, /)
 |      Return divmod(value, self).
 |  
 |  __repr__(self, /)
 |      Return repr(self).
 |  
 |  __rfloordiv__(self, value, /)
 |      Return value//self.
 |  
 |  __rlshift__(self, value, /)
 |      Return value<<self.
 |  
 |  __rmod__(self, value, /)
 |      Return value%self.
 |  
 |  __rmul__(self, value, /)
 |      Return value*self.
 |  
 |  __ror__(self, value, /)
 |      Return value|self.
 |  
 |  __round__(...)
 |      Rounding an Integral returns itself.
 |      Rounding with an ndigits argument also returns an integer.
 |  
 |  __rpow__(self, value, mod=None, /)
 |      Return pow(value, self, mod).
 |  
 |  __rrshift__(self, value, /)
 |      Return value>>self.
 |  
 |  __rshift__(self, value, /)
 |      Return self>>value.
 |  
 |  __rsub__(self, value, /)
 |      Return value-self.
 |  
 |  __rtruediv__(self, value, /)
 |      Return value/self.
 |  
 |  __rxor__(self, value, /)
 |      Return value^self.
 |  
 |  __sizeof__(...)
 |      Returns size in memory, in bytes
 |  
 |  __str__(self, /)
 |      Return str(self).
 |  
 |  __sub__(self, value, /)
 |      Return self-value.
 |  
 |  __truediv__(self, value, /)
 |      Return self/value.
 |  
 |  __trunc__(...)
 |      Truncating an Integral returns itself.
 |  
 |  __xor__(self, value, /)
 |      Return self^value.
 |  
 |  bit_length(...)
 |      int.bit_length() -> int
 |      
 |      Number of bits necessary to represent self in binary.
 |      >>> bin(37)
 |      '0b100101'
 |      >>> (37).bit_length()
 |      6
 |  
 |  conjugate(...)
 |      Returns self, the complex conjugate of any int.
 |  
 |  from_bytes(...) from builtins.type
 |      int.from_bytes(bytes, byteorder, *, signed=False) -> int
 |      
 |      Return the integer represented by the given array of bytes.
 |      
 |      The bytes argument must either support the buffer protocol or be an
 |      iterable object producing bytes.  Bytes and bytearray are examples of
 |      built-in objects that support the buffer protocol.
 |      
 |      The byteorder argument determines the byte order used to represent the
 |      integer.  If byteorder is 'big', the most significant byte is at the
 |      beginning of the byte array.  If byteorder is 'little', the most
 |      significant byte is at the end of the byte array.  To request the native
 |      byte order of the host system, use `sys.byteorder' as the byte order value.
 |      
 |      The signed keyword-only argument indicates whether two's complement is
 |      used to represent the integer.
 |  
 |  to_bytes(...)
 |      int.to_bytes(length, byteorder, *, signed=False) -> bytes
 |      
 |      Return an array of bytes representing an integer.
 |      
 |      The integer is represented using length bytes.  An OverflowError is
 |      raised if the integer is not representable with the given number of
 |      bytes.
 |      
 |      The byteorder argument determines the byte order used to represent the
 |      integer.  If byteorder is 'big', the most significant byte is at the
 |      beginning of the byte array.  If byteorder is 'little', the most
 |      significant byte is at the end of the byte array.  To request the native
 |      byte order of the host system, use `sys.byteorder' as the byte order value.
 |      
 |      The signed keyword-only argument determines whether two's complement is
 |      used to represent the integer.  If signed is False and a negative integer
 |      is given, an OverflowError is raised.
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  denominator
 |      the denominator of a rational number in lowest terms
 |  
 |  imag
 |      the imaginary part of a complex number
 |  
 |  numerator
 |      the numerator of a rational number in lowest terms
 |  
 |  real
 |      the real part of a complex number

Exercise: String functions

  • Use the <str>.replace() function replace the text in "Hello world" to "Hello python"
  • Use the <str>.zfill() function to take an integer and pad it with zeroes to make it 5 characters long: Define a = 512 and convert it to "00512"
  • Use the <str>.startswith() function to check if a string starts with a vowel. Run it on "python" and "india".
help("abc".zfill(1))
Help on module abc:

NAME
    abc - Abstract Base Classes (ABCs) according to PEP 3119.

MODULE REFERENCE
    http://docs.python.org/3.4/library/abc
    
    The following documentation is automatically generated from the Python
    source files.  It may be incomplete, incorrect or include features that
    are considered implementation detail and may vary between Python
    implementations.  When in doubt, consult the module reference at the
    location listed above.

CLASSES
    builtins.classmethod(builtins.object)
        abstractclassmethod
    builtins.object
        ABC
    builtins.property(builtins.object)
        abstractproperty
    builtins.staticmethod(builtins.object)
        abstractstaticmethod
    builtins.type(builtins.object)
        ABCMeta
    
    class ABC(builtins.object)
     |  Helper class that provides a standard way to create an ABC using
     |  inheritance.
     |  
     |  Data descriptors defined here:
     |  
     |  __dict__
     |      dictionary for instance variables (if defined)
     |  
     |  __weakref__
     |      list of weak references to the object (if defined)
     |  
     |  ----------------------------------------------------------------------
     |  Data and other attributes defined here:
     |  
     |  __abstractmethods__ = frozenset([])
    
    class ABCMeta(builtins.type)
     |  Metaclass for defining Abstract Base Classes (ABCs).
     |  
     |  Use this metaclass to create an ABC.  An ABC can be subclassed
     |  directly, and then acts as a mix-in class.  You can also register
     |  unrelated concrete classes (even built-in classes) and unrelated
     |  ABCs as 'virtual subclasses' -- these and their descendants will
     |  be considered subclasses of the registering ABC by the built-in
     |  issubclass() function, but the registering ABC won't show up in
     |  their MRO (Method Resolution Order) nor will method
     |  implementations defined by the registering ABC be callable (not
     |  even via super()).
     |  
     |  Method resolution order:
     |      ABCMeta
     |      builtins.type
     |      builtins.object
     |  
     |  Methods defined here:
     |  
     |  __instancecheck__(cls, instance)
     |      Override for isinstance(instance, cls).
     |  
     |  __subclasscheck__(cls, subclass)
     |      Override for issubclass(subclass, cls).
     |  
     |  register(cls, subclass)
     |      Register a virtual subclass of an ABC.
     |      
     |      Returns the subclass, to allow usage as a class decorator.
     |  
     |  ----------------------------------------------------------------------
     |  Static methods defined here:
     |  
     |  __new__(mcls, name, bases, namespace)
     |  
     |  ----------------------------------------------------------------------
     |  Methods inherited from builtins.type:
     |  
     |  __call__(self, /, *args, **kwargs)
     |      Call self as a function.
     |  
     |  __delattr__(self, name, /)
     |      Implement delattr(self, name).
     |  
     |  __dir__(...)
     |      __dir__() -> list
     |      specialized __dir__ implementation for types
     |  
     |  __getattribute__(self, name, /)
     |      Return getattr(self, name).
     |  
     |  __init__(self, /, *args, **kwargs)
     |      Initialize self.  See help(type(self)) for accurate signature.
     |  
     |  __prepare__(...) from builtins.type
     |      __prepare__() -> dict
     |      used to create the namespace for the class statement
     |  
     |  __repr__(self, /)
     |      Return repr(self).
     |  
     |  __setattr__(self, name, value, /)
     |      Implement setattr(self, name, value).
     |  
     |  __sizeof__(...)
     |      __sizeof__() -> int
     |      return memory consumption of the type object
     |  
     |  __subclasses__(...)
     |      __subclasses__() -> list of immediate subclasses
     |  
     |  mro(...)
     |      mro() -> list
     |      return a type's method resolution order
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors inherited from builtins.type:
     |  
     |  __abstractmethods__
     |  
     |  __dict__
     |  
     |  __text_signature__
     |  
     |  ----------------------------------------------------------------------
     |  Data and other attributes inherited from builtins.type:
     |  
     |  __base__ = <class 'type'>
     |      type(object_or_name, bases, dict)
     |      type(object) -> the object's type
     |      type(name, bases, dict) -> a new type
     |  
     |  __bases__ = (<class 'type'>,)
     |  
     |  __basicsize__ = 824
     |  
     |  __dictoffset__ = 264
     |  
     |  __flags__ = 2148292097
     |  
     |  __itemsize__ = 40
     |  
     |  __mro__ = (<class 'abc.ABCMeta'>, <class 'type'>, <class 'object'>)
     |  
     |  __weakrefoffset__ = 368
    
    class abstractclassmethod(builtins.classmethod)
     |  A decorator indicating abstract classmethods.
     |  
     |  Similar to abstractmethod.
     |  
     |  Usage:
     |  
     |      class C(metaclass=ABCMeta):
     |          @abstractclassmethod
     |          def my_abstract_classmethod(cls, ...):
     |              ...
     |  
     |  'abstractclassmethod' is deprecated. Use 'classmethod' with
     |  'abstractmethod' instead.
     |  
     |  Method resolution order:
     |      abstractclassmethod
     |      builtins.classmethod
     |      builtins.object
     |  
     |  Methods defined here:
     |  
     |  __init__(self, callable)
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors defined here:
     |  
     |  __weakref__
     |      list of weak references to the object (if defined)
     |  
     |  ----------------------------------------------------------------------
     |  Data and other attributes defined here:
     |  
     |  __isabstractmethod__ = True
     |  
     |  ----------------------------------------------------------------------
     |  Methods inherited from builtins.classmethod:
     |  
     |  __get__(self, instance, owner, /)
     |      Return an attribute of instance, which is of type owner.
     |  
     |  __new__(*args, **kwargs) from builtins.type
     |      Create and return a new object.  See help(type) for accurate signature.
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors inherited from builtins.classmethod:
     |  
     |  __dict__
     |  
     |  __func__
    
    class abstractproperty(builtins.property)
     |  A decorator indicating abstract properties.
     |  
     |  Requires that the metaclass is ABCMeta or derived from it.  A
     |  class that has a metaclass derived from ABCMeta cannot be
     |  instantiated unless all of its abstract properties are overridden.
     |  The abstract properties can be called using any of the normal
     |  'super' call mechanisms.
     |  
     |  Usage:
     |  
     |      class C(metaclass=ABCMeta):
     |          @abstractproperty
     |          def my_abstract_property(self):
     |              ...
     |  
     |  This defines a read-only property; you can also define a read-write
     |  abstract property using the 'long' form of property declaration:
     |  
     |      class C(metaclass=ABCMeta):
     |          def getx(self): ...
     |          def setx(self, value): ...
     |          x = abstractproperty(getx, setx)
     |  
     |  'abstractproperty' is deprecated. Use 'property' with 'abstractmethod'
     |  instead.
     |  
     |  Method resolution order:
     |      abstractproperty
     |      builtins.property
     |      builtins.object
     |  
     |  Data descriptors defined here:
     |  
     |  __dict__
     |      dictionary for instance variables (if defined)
     |  
     |  __weakref__
     |      list of weak references to the object (if defined)
     |  
     |  ----------------------------------------------------------------------
     |  Data and other attributes defined here:
     |  
     |  __isabstractmethod__ = True
     |  
     |  ----------------------------------------------------------------------
     |  Methods inherited from builtins.property:
     |  
     |  __delete__(self, instance, /)
     |      Delete an attribute of instance.
     |  
     |  __get__(self, instance, owner, /)
     |      Return an attribute of instance, which is of type owner.
     |  
     |  __getattribute__(self, name, /)
     |      Return getattr(self, name).
     |  
     |  __init__(self, /, *args, **kwargs)
     |      Initialize self.  See help(type(self)) for accurate signature.
     |  
     |  __new__(*args, **kwargs) from builtins.type
     |      Create and return a new object.  See help(type) for accurate signature.
     |  
     |  __set__(self, instance, value, /)
     |      Set an attribute of instance to value.
     |  
     |  deleter(...)
     |      Descriptor to change the deleter on a property.
     |  
     |  getter(...)
     |      Descriptor to change the getter on a property.
     |  
     |  setter(...)
     |      Descriptor to change the setter on a property.
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors inherited from builtins.property:
     |  
     |  fdel
     |  
     |  fget
     |  
     |  fset
    
    class abstractstaticmethod(builtins.staticmethod)
     |  A decorator indicating abstract staticmethods.
     |  
     |  Similar to abstractmethod.
     |  
     |  Usage:
     |  
     |      class C(metaclass=ABCMeta):
     |          @abstractstaticmethod
     |          def my_abstract_staticmethod(...):
     |              ...
     |  
     |  'abstractstaticmethod' is deprecated. Use 'staticmethod' with
     |  'abstractmethod' instead.
     |  
     |  Method resolution order:
     |      abstractstaticmethod
     |      builtins.staticmethod
     |      builtins.object
     |  
     |  Methods defined here:
     |  
     |  __init__(self, callable)
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors defined here:
     |  
     |  __weakref__
     |      list of weak references to the object (if defined)
     |  
     |  ----------------------------------------------------------------------
     |  Data and other attributes defined here:
     |  
     |  __isabstractmethod__ = True
     |  
     |  ----------------------------------------------------------------------
     |  Methods inherited from builtins.staticmethod:
     |  
     |  __get__(self, instance, owner, /)
     |      Return an attribute of instance, which is of type owner.
     |  
     |  __new__(*args, **kwargs) from builtins.type
     |      Create and return a new object.  See help(type) for accurate signature.
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors inherited from builtins.staticmethod:
     |  
     |  __dict__
     |  
     |  __func__

FUNCTIONS
    abstractmethod(funcobj)
        A decorator indicating abstract methods.
        
        Requires that the metaclass is ABCMeta or derived from it.  A
        class that has a metaclass derived from ABCMeta cannot be
        instantiated unless all of its abstract methods are overridden.
        The abstract methods can be called using any of the normal
        'super' call mechanisms.
        
        Usage:
        
            class C(metaclass=ABCMeta):
                @abstractmethod
                def my_abstract_method(self, ...):
                    ...
    
    get_cache_token()
        Returns the current ABC cache token.
        
        The token is an opaque object (supporting equality testing) identifying the
        current version of the ABC cache for virtual subclasses. The token changes
        with every call to ``register()`` on any ABC.

FILE
    /srv/paws/lib/python3.4/abc.py


5. If Else

Like all languages, Python allows us to conditionally run code.

To have an if condition we need the idea of something being true and something being false. We have True or False as "boolean" values. True would represent OK where as false would represent No or Cancel.

False is False
True
True is False
False
true is False
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-24-16525f6953e7> in <module>()
----> 1 true is False

NameError: name 'true' is not defined

We can write expressions with operations too.

1 > 2
False
"Cool".startswith("C")
True
"Cool".endswith("C")
False
"oo" in "Cool"
True
42 == 1 # note the double equals sign for equality
False

In order to write an "if" statement we need code that spans multiple lines

if condition:
    print("Condition is True")
else:
    print("Condition is False")

Some things to notice. The if condition ends in a colon (":"). In Python blocks of code are indicated with a colon (":") and are grouped by white space. Notice the else also ends with a colon (":"), "else:". Let's try changing the condition and see what happens.

condition = (1 > 2)
if condition:
    print("Condition is True")
else:
    print("Condition is False")
Condition is False

About that white space, consider the following code:

if condition:
    print("Condition is True")
else:
    print("Condition is False")
print("Condition is True or False, either way this is outputted")

Since the last print statement isn't indented it gets run after the if block or the else block.

You can play with this. Try indenting the last print statement below and see what happens.

condition = True
if condition:
    print("Condition is True")
else:
    print("Condition is False")
print("Condition is True or False, either way this is outputted")
Condition is True
Condition is True or False, either way this is outputted

You can also use "and", "or", "not" to combine conditions (No ugly && and || here):

True and True is True
True and False is False
False and True is False
False and False is False
not True is False

Exercise - Boolean values

Below change the values of the three variables to make the entire "if condition" true.

# Edit the values of these 3 variables
boolean_literal = True
number = 11
string_literal = "I like to count cows before bed."

# Don't change the code below this
if number > 10 and boolean_literal and "cows" in string_literal:
    print("Success!")
else:
    print("Try again!")
Success!

6. Lists

So far we have numbers, strings, and conditional if statements. Now for our first container - a list.

A list in Python is just like an array or a linked list. They have a defined order and you can add to it or remove from it. Let's take a look at some simple lists.

# The empty list
empty_list = []
print(empty_list)
[]
breakfast_list = ["bread", "butter", "jam"]
print(breakfast_list)
['bread', 'butter', 'jam']
[1,2,3]
[1, 2, 3]

List literals are all about square brackets ("[ ]") and commas (","). You can create a list of literals by wrapping them in square brackets and separating them with commas.

As python doesn't care too much about data types, you can even mix different types of things into the same list; numbers, strings, booleans (unlike arrays)

[True, 0, "String"]
[True, 0, 'String']

We can put variables into a list and set a variable to a list.

wikipedia = "wikipedia.org"
wiki_sites_list = ["wikidata.org", wikipedia]
print(wiki_sites_list)
['wikidata.org', 'wikipedia.org']

Like strings, lists have their own operations. "append" is an interesting one. "append" lets you add an item to the end of a list.

wiki_sites_list = ["wikidata.org", "wikipedia.org"]
wiki_sites_list.append("wikisource.org")
print(wiki_sites_list)
['wikidata.org', 'wikipedia.org', 'wikisource.org']
wiki_sites_list[0]
'wikidata.org'

There is that 0 indexing again. The first element of the list is given index value 0.

print("These are some of the wikimedia sites: {0}, {1}, {2}".format(wiki_sites_list[0], wiki_sites_list[1], wiki_sites_list[2]))
These are some of the wikimedia sites: wikidata.org, wikipedia.org, wikisource.org

Length

We can also find the lenght of the list using the len function:

len(wiki_sites_list)
3

Splicing

Parts of a list can also be gotten using the splice operator. To use the splice operator, square brackets with color (:) is used:

nums = [0,1,2,3,4,5]
print("2:4 ->", nums[2:4])  # Takes everything 2nd to 3rd term
print("1:3 ->", nums[1:3])  # Takes everything from 1st to 2nd term
print("2: ->", nums[2:])    # Takes everything from 2nd to last term
print(":4 ->", nums[:4])    # Takes everything from start to the 3rd term
print(":-2 ->", nums[:-2])  # Takes everything from start to the 3rd last term
2:4 -> [2, 3]
1:3 -> [1, 2]
2: -> [2, 3, 4, 5]
:4 -> [0, 1, 2, 3]
:-2 -> [0, 1, 2, 3]

Exercise - Use the functions .join() and .split()

  • Use the <str>.join() function to join a list of items using a comma: Convert ["bread", "butter", "jam"] -> "bread, butter, jam"
  • Use the <str>.split() function to split a string into a list: Convert back the earlier "bread, butter, jam" -> ["bread", "butter", "jam"]
help(str.join)
Help on method_descriptor:

join(...)
    S.join(iterable) -> str
    
    Return a string which is the concatenation of the strings in the
    iterable.  The separator between elements is S.

7. Loops

Indexes are useful, but lists really shine when you start looping.

Loops let you do something for each item in a list. They look like this:

for item in my_list:
    print(item)  # Do any action per item in the list

"for" and "in" are required. "my_list" can be any variable or literal which is like a list. "item" is the name you want to give each item of the list in the indented block as you iterate through. We call each step where item has a new value an iteration.

for num in [1, 2, 3]:
    print(num)
1
2
3

In python, we don't use constructs like for ( int i = 0; i < maximum; i++ ) which are just confusing, rather we create a "range" of integers from 0 to maximum and loop over that:

maximum = 10
print("This is the range object:", range(0, maximum))
print("This is what it creates when it's used in a loop or other functions:", list(range(0, maximum)))
This is the range object: range(0, 10)
This is what it creates when it's used in a loop or other functions: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

And then we can use that with a loop to print a list of squares.

for number in range(0, 10):
    print("{0} squared is {1}".format(number, number*number))
0 squared is 0
1 squared is 1
2 squared is 4
3 squared is 9
4 squared is 16
5 squared is 25
6 squared is 36
7 squared is 49
8 squared is 64
9 squared is 81

8. Dictionaries

We have come a long way! Just one more section. Dictionaries are another container like lists, but instead of being index by a number like 0 or 1 it is indexed by a key which can be almost anything. The name comes from being able to use it to represent a dictionary.

List literals use square brackets ("[]") but dictionaries use braces ("{}").

{"wikipedia.org": "The free encyclopedia", 
 "wikisource.org": "The free library"}

In a dictionary the key comes first followed by a colon (":") than the value then a comma (",") then another key and so on. This is one situation where a colon doesn't start a block.

wiki_sites = {
    "wikipedia.org": "The free encyclopedia", 
    "wikisource.org": "The free library"
}
wiki_sites["wikipedia.org"]
'The free encyclopedia'

We can loop over the keys in a dictionary to list all of our definitions...

for key in wiki_sites:
    print('The Key is "{0}" and the value is "{1}"'.format(key, wiki_sites[key]))
The Key is "wikipedia.org" and the value is "The free encyclopedia"
The Key is "wikisource.org" and the value is "The free library"

In fact, dictionaries can contain any type of values. Hence, you can have a list as the value of a dictionary:

wiki_language_sites = {
    "wikipedia.org": ["en.wikipedia.org", "ml.wikipedia.org", "ta.wikipedia.org"], 
    "wikisource.org": ["en.wikisource.org", "ml.wikisource.org", "ta.wikisource.org"]
}
wiki_language_sites["wikipedia.org"]
['en.wikipedia.org', 'ml.wikipedia.org', 'ta.wikipedia.org']

We can also get the keys and values as lists:

print("Keys:", list(wiki_sites.keys()))
print("Values:", list(wiki_sites.values()))
Keys: ['wikipedia.org', 'wikisource.org']
Values: ['The free encyclopedia', 'The free library']

Exercise: Loop over the keys, values, and items

  • Loop over a dict using for key in <dict>: and print all the keys. Verify that the same result is got when using for key in <dict>.keys().
  • Loop over the values of a dict by first getting all the values using the <dict>.values() function.
  • Loop over the keys and values of a dict by requesting 2 items from the list. Do for key, val in dict.items(): and try printing both the key and value side by side.

9. Functions

To avoid code repetition and to break code into smaller intelligible blocks, functions are useful. A function takes in certain variables (arguments) and gives out a return value. The arguments and return value do not need to define a data type again, as python isn't too worried about data types.

A function is defined using the def keyword:

def <function name>(<arg1>, <arg2>, <arg3>, ..., <argN>):
    <function body>
    return <value to return>
def sum2(x1, x2):
    return x1 + x2

print("2 + 3 =", sum2(2, 3))
print('"a" + "b" =', sum2("a", "b"))
print('[1] + [2,3] =', sum2([1], [2, 3]))
2 + 3 = 5
"a" + "b" = ab
[1] + [2,3] = [1, 2, 3]

Because there's no data type for the arguments and return type, we can give strings, ints, floats, lists, etc. Anything that supports the + operator in our example. But if the arguments given cannot be operated on by + it gives an error. And it is the job of the developer of the function to ensure that the arguments given are sane:

def sum2(x1, x2):
    return x1 + x2

print('1 + "s" =', sum2(1, "s"))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-52-8cc7b8e2b21d> in <module>()
      2     return x1 + x2
      3 
----> 4 print('1 + "s" =', sum2(1, "s"))

<ipython-input-52-8cc7b8e2b21d> in sum2(x1, x2)
      1 def sum2(x1, x2):
----> 2     return x1 + x2
      3 
      4 print('1 + "s" =', sum2(1, "s"))

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

Python also supports assigning values using the argument name. When using argument names (also called keyword arguments - kwargs) the order of the arguments do not matter as the name of the argument is used. This gives flexibility, as you don't have to remember which argument comes first and which comes second! Let's see it in action:

def difference2(x1, x2):
    return x1 - x2

print("2 - 3 =", difference2(2, 3))
print("2 - 3 =", difference2(x1=2, x2=3))
print("2 - 3 =", difference2(x2=3, x1=2))  # We give x2 before x1
2 - 3 = -1
2 - 3 = -1
2 - 3 = -1

Not all functions need to return a value. And by default if a return value is not given, it returns None.

Functions can also have default values for arguments:

def power(base, exponent=2):
    return base ** exponent

print('5 power -1 is', power(5, -1))
print('power(5) uses exponent 2 by default:', power(5))
5 power -1 is 0.2
power(5) uses exponent 2 by default: 25

Exercise - Fibonacci function

Write a function that prints the values of the fibonacci series until a given maximum value. For example, print_fibonacci(10) would print:

>>> print_fibonacci(10)
1, 1, 2, 3, 5, 8
a=0
b=1
def fibonacci(number):

    if c < number:
        c = a + b
        a = b
        b = c
        print (c + ",")
    
fibonacci(10)
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-63-0a7425d2c8c7> in <module>()
      9         print (c + ",")
     10 
---> 11 fibonacci(10)

<ipython-input-63-0a7425d2c8c7> in fibonacci(number)
      3 def fibonacci(number):
      4 
----> 5     if c < number:
      6         c = a + b
      7         a = b

UnboundLocalError: local variable 'c' referenced before assignment

10. Classes

Python also has Object Oriented Programming (OOP) in it's structure. A class is a template which holds class variables and functions (methods) and operates on the class itself.

To use a class, objects which conform to the class template need to be created:

class WebSite:
    def __init__(self):
        self.url = ""
        self.description = ""

w1 = WebSite()  # w1 is an object of class WebSite
print("Type of w1:", type(w1))
w1.description = "The free encyclopedia"
w1.url = "wikipedia.org"
Type of w1: <class '__main__.WebSite'>

The __init__() function is the class constructor and can not return anything. The first argument self is the object that's being created. Hence, in our above class, when the object is created, it has 2 member variables url and description which are assigned to empty string ("") in the constructor.

Other than the variables created in the class, python can dynamically add more variables in the class:

w1.visits = 1000
w1.subsites = ["en.wikipedia.org", "ml.wikipedia.org", "ta.wikipedia.org"]
print(w1.visits, w1.subsites)
1000 ['en.wikipedia.org', 'ml.wikipedia.org', 'ta.wikipedia.org']

Classes can also define functions which can be used to perform functions using the object variables.

Object methods start with the keyword self normally. When w1.function() is used, the object before the period (w1) is passed to function()'s first argument. Hence, function() would be defined as def function(self): where self is a reference to the object itself.

Here's an example:

class WebSite:
    def __init__(self, description="", url=""):
        self.url = url
        self.description = description

    # A class method or a class function which can be called usingt the object using: <obj>.subsite("arg")
    # Which the class receives as: substitute(<obj>, "arg")
    def subsite(self, subname):
        return subname + "." + self.url

w1 = WebSite(description="The free encyclopedia", url="wikipedia.org")  # w1 is an object of class WebSite
w1.subsites = [w1.subsite('en'), w1.subsite('ml'), w1.subsite('ta')]
print(w1.subsites)
['en.wikipedia.org', 'ml.wikipedia.org', 'ta.wikipedia.org']

Exercise - Create a WebSite object for each item in the wiki_sites dictionary

Loop over every (key, value) pair in the dictionary wiki_sites and create a list of WebSite objects using the WebSite class.

  • The key of the dictionary should be stored in the url of the object.
  • The value of the dictionary should be stored in the description of the object.
# Leave the definitions of the class and dictionary as is
class WebSite:
    def __init__(self, description="", url=""):
        self.url = url
        self.description = description

wiki_sites = {
    "wikipedia.org": "The free encyclopedia", 
    "wikisource.org": "The free library"
}

# Write below. Loop over every item in wiki_sites and create a list of WebSite objects.

Exercise - Create a class to store a mediawiki page

Create a class which can store a mediawiki page (has content and name of the page) and create 2 functions to:

  1. Check if the page is empty (the content is an empty string ""): <obj>.empty() which returns True or False
  2. Generate the URL of the page using a base url: <obj>.url() returns "wikipedia.org/wiki/<page name>")

Below you will find some example code to test your class

# Add the class definition here

# The following should work:
page = WikiPage(name="MyPage", content="Hello there ! This is a small example page.")
print("The name of the page is:", page.name)
print("The page has the following content:")
print(page.content)
print("The url of the page is:", page.url())
if page.empty():
    print("The page is empty")
else:
    print("The page is not empty")
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-69-b0dc2c497325> in <module>()
      2 
      3 # The following should work:
----> 4 page = WikiPage(name="MyPage", content="Hello there ! This is a small example page.")
      5 print("The name of the page is:", page.name)
      6 print("The page has the following content:")

NameError: name 'WikiPage' is not defined