Chapter 2 - Introduction to Python

1. Python

Python is a programming language that has been under development for over 25 years [1].

1.1 Statements

Python is an imperative language based on statements. That is, programs in Python consists of lines composed of statements. A statement can be:

a single expression
an assignment
a function call
a function definition
a statement; statement

1.1.1 Expressions

Numbers
- integers
- floating-point
- complex numbers
strings
boolean values
lists

1.1.1.1 Numbers



In [1]:

    
1









    Out[1]:





1



In [ ]:

    
2



In [10]:

    
-3









    Out[10]:





-3



In [2]:

    
1
2









    Out[2]:





2



In [5]:

    
3.14









    Out[5]:





3.14

1.1.1.2 Strings



In [7]:

    
'apple'









    Out[7]:





'apple'



In [8]:

    
"apple"









    Out[8]:





'apple'

Notice that the Out might not match exactly the In. In the above example, we used double-quotes but the representation of the string used single-quotes. Python will default to showing representations of values using single-quotes, if it can.

1.1.1.3 Boolean Values



In [21]:

    
True









    Out[21]:





True



In [22]:

    
False









    Out[22]:





False

1.1.1.4 Lists

List, and a read-only list (called a tuple).



In [11]:

    
[1, 2, 3]









    Out[11]:





[1, 2, 3]



In [23]:

    
(1, 2, 3)









    Out[23]:





(1, 2, 3)



In [24]:

    
1, 2, 3









    Out[24]:





(1, 2, 3)

1.1.2 Function Calls

There are two ways to call functions in Python:

by pre-defined infix operator name
by function name, followed by parentheses

Infix operator name:



In [4]:

    
1 + 2









    Out[4]:





3



In [9]:

    
abs(-1)









    Out[9]:





1

1.1.2.1 Print

Evaluating and display result as an Out, versus evaluating and printing result (side-effect).



In [3]:

    
print(1)

1.1.3 Special Values



In [12]:

    
None

1.1.4 Defining Functions



In [16]:

    
def plus(a, b):
    return a + b



In [17]:

    
plus(3, 4)









    Out[17]:





7



In [18]:

    
def plus(a, b):
    a + b



In [19]:

    
plus(3, 4)

What happened? All functions return something, even if you don't specify it. If you don't specify a return value, then it will default to returning None.



In [1]:

    
"a" + 1









    



---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-1-bbb7406f1324> in <module>()
----> 1 "a" + 1

TypeError: Can't convert 'int' object to str implicitly

Sidebar 2-1: How to Read Python Error Messages

Python error messages

TypeError: Can't convert 'int' object to str implicitly

Above the error message is the "traceback" also called the "call stack". This is a representation of the sequence of procedure calls that lead to the error. If the procedure call originated from code from a file, the filename would be listed after the word "File" on each line. If the procedure call originated from a notebook cell, then the word "ipython-input-#-HEX".

1.2 Equality

1.2.2 ==



In [2]:

    
1 == 1









    Out[2]:





True

1.2.1 is



In [4]:

    
[] is []









    Out[4]:





False



In [6]:

    
list() is list()









    Out[6]:





False



In [5]:

    
tuple() is tuple()









    Out[5]:





True



In [7]:

    
57663463467 is 57663463467









    Out[7]:





True

References

[1] https://en.wikipedia.org/wiki/History_of_Python



In [ ]: