Chapter 1: Python Basics

Running the interpreter #

Executing the interpreter without arguments runs the REPL or interactive command line:

$ python
>>>

To execute a command immediately use the -c flag:

$ python -c 'print("Hello world")'
Hello world

The interpreter can also take in a file to execute commands on:

# sample.py
print("Hello from file!")

$ python sample.py
Hello from file!

To run a python file and then immediately go into REPL run with the -i flag:

$ python -i sample.py
Hello from file!
>>>

Basic I/O #

Printing #

The print function is used to print output to stdout.

print("Hello world")

Arguments #

Arguments are provided through the sys standard module, specifically on the argv list within it.

To use a module the import <module_name> syntax must be used:

# args.py
import sys
print(sys.argv)

$ python args.py hello there
['args.py', 'hello', 'there']

Arithmetic #

# Variable declarations
x = 5
y = 2

# Basic Arithmetic
x + y
x - y
x * y
x / y
x % y

# Other Arithmetic Operations

# Floor division
x // y

# Exponents
x ** y

Note: An operation between an integer and a float in Python will always result as a float and will always have the integer operand get casted to a float.

Strings #

Zero or more characters in between '' or "" are denoted as strings.

Concatenation & Repetition #

Strings can be concatenated using the + operator:

>>> Hello = "Hello"
>>> World = "World"
>>> print(Hello + " " + World)
'Hello World'

Python also automatically concatenates strings that are adjacent to each other:

>>> 'Pre' 'fix'
'Prefix'

This is useful if you want to break long strings:

>>> my_string = ("The quick brown fox jumps over "
...              "the lazy dog")
>>> print(my_string)
'The quick brown fox jumps over the lazy dog'

A string can be repeated n times by using a * operator to it with an integer.

>>> 3 * 'super'
'supersupersuper'

Indexing/Slicing #

Strings can be indexed using a single integer as long as it is within range:

>>> str = 'Batman'
>>> str[0]
'B'
>>> str[1]
'a'
>>> str[2]
't'
>>> str[5]
'n'
>>> str[6]
Error!

Negative integers can also be used, in this case it will get subtracted to the length of the string:

>>> str[-1]
'n'
>>> str[-2]
'a'
>>> str[-3]
'm'

Python also provides a syntax to slice or retrieve a substring using the range syntax a:b where $a \leq b \leq S.length$ and $S$ is a string.

>>> str[0:3]
'Bat'
>>> str[3:6]
'man'

Omiting the end of the range implies an index equivalent to the length of the string:

>>> str[3:]
'man'
>>> 'C' + str[1:]
'Catman'

Conversely, omiting the start of the range implies an index of 0:

>>> str[:3] + 'girl'
'Batgirl'

If any of the integers is beyond the bounds of the string when using range indices, python will handle it gracefully:

>>> str[3:69]
'man'
>>> str[69:420]
''

Lists #

A list is a data structure that contain elements in a contiguous manner:

>>> my_list = [1, 'apples', 23, 69, 'orange']
>>> print(my_list)
[1, 'apples', 23, 69, 'orange']