Sherman's One-Hour Python Tutorial

Let's learn some basics of python in 60 minutes.

We'll assume you already have python 2.7 installed. The code here should work in 3.x as well. There are some differences which can break your code if you use the wrong version.

Running Python Interactively

The python command starts the python interpreter.

bash$ python

You will see the following prompt:

.hljs[

Python 2.7.12 (default, Oct 11 2016, 05:24:00)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.38)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

]

--

Now try telling python to do something:

>>> print("hello world")
hello world
>>>

Running a Python Script

This is a simple python script, which we'll save as tut1.py:

#!/usr/bin/env python
print("hello world")

--

Make the script executable:

bash$ chmod +x tut1.py
bash$

--

Run the script from the command prompt:

bash$ ./tut1.py
hello world
bash$

--

You have just written and executed your first python program!

Variables in Python

You create a variable just by assigning to it. No var or other instantiation keywords. No declaration, just assign.

x = 7
y = "foo"
pi = 3.14159265

Numbers in Python

Integers:

x = 10
y = 3
print(x/y) # prints 3 (python 2.7)
           # prints 3.3333333333333335 (python 3)

Notice that integer division gives you an integer in 2.7, but a float in 3.x.

--

Floating Point:

x = 3.14159265
y = 7
print(x/y) # prints 0.44879895000000003

Control Flow

Blocks are defined in python by indenting the code.

The if statement executes the indented code block if the condition evaluates to True:

if condition:
    statement
elif condition:
    statement
else:
    statement

--

Another form of if is the ternary operator:

y = 7
x = 5 if y>1 else 10 # x is now equal to 5

Loops

The for loop iterates over a sequence:

for name in [ "bob", "sam", "ben", "sue", "bev" ]:
    print(name)

--

You can use the range function to use a numeric counter:

for i in range(10):
    print(i) # prints 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

--

The while loop iterates as long as a condition is true:

i = 0
while i < 10:
    print(i)
    i = i+1

Exiting a Loop

The continue statement skips to the next iteration of the loop and continues looping:

for i in range(1, 11):
    if i == 3:
        continue
    print(i) # prints 1 2 4 5 6 7 8 9 10

--

The break statement skips out of the loop without executing any more iterations.

for i in range(10):
    if i == 3:
        break
    print(i) # prints 0 1 2

Else

The else is not just used after if, but can follow a loop:

for i in range(10):
    pass # this statement does nothing! use it as a placeholder
else:
    print("else")

--

The else block is only executed after the loop condition evaluates as False, but not following a break:

for i in range(10):
    if i == 3:
        break
else:
    print("else")   # this line is skipped

print("done")       # prints "done"

Sequence Types

There are six sequence types in python: str, unicode, list, tuple, buffer, xrange. We are going to talk about str, list, tuple.

--

Access elements of a sequence (such as a string) using brackets. The indexes start with zero:

x = "bar"
print(x[0]) # b
print(x[1]) # a
print(x[2]) # r

This may look familiar like arrays in other languages.

--

Strings are written using single or double quotes. Strings with double quotes can contain apostrophes or single quotes. Strings with single quotes can contain double quotes.

x = "this is a test"
print( len(x) ) # prints 14
print( x[10] )  # prints t
y = "we're home!"
z = 'sam says "hi"'

Lists and Tuples

Lists are constructed with brackets and every element should be the same type. Lists can be changed and appened (mutable).

mylist = [ "foo", "bar", "baz", "wow" ]
mylist[4] = "new"
print( mylist[4] ) # prints "new"

--

Tuples are created with parentheses. They can not be appended nor can elements be replaced (immutable). You would have to create a new tuple with different elements instead.

mytuple = (1, "foo", 3.14159265)
print( mytuple[1] ) # prints "foo"
mytuple[1] = "bar" # TypeError: 'tuple' object does not support item assignment

Slices

seq = [ "apple", "banana", "pear", "grape", "plum" ]

Slice seq[ start : stop ] copies a sequence from the original sequence, where start is the index of the first element to copy, and stop is the index of the element after the last element to copy.

seq[1:3] # ['banana', 'pear']

--

Negative indexes count backwards from the end of the list. This works for indexing single elements or slices:

seq[1:-1] # ['banana', 'pear', 'grape']
seq[-3:3] # ['pear']
seq[-1:1] # []

--

You may omit either index in the slice to read from the start or to the end of the list:

seq[2:]  # ['pear', 'grape', 'plum']
seq[:-3] # ['apple', 'banana']
seq[:]   # copy the entire list!

Dictionaries

A dictionary maps keys to values. It is an unordered collection of key/value pairs.

d = { "foo": 37, "bar": 99, "baz": 42, "woo": 0 }

--

Access a key using bracket notation. You can add new keys this way too:

d["foo"] # 37
d["what"] = 45

--

Remove a key using del operator:

del d["bar"]

--

Use the in operator to check if a key exists:

"foo" in d # True
"noo" in d # False

Comprehensions

A list comprehension is a construct that generates a new list by transforming a list.

The syntax: [ expression for item in list if conditional ]

[x+1 for x in range(10) if x % 2 == 0] # [ 1, 3, 5, 7, 9 ]

Functions

Define a function using def. Returning a value works similarly to other languages:

def cube( x ):
    return x * x * x

Function vs Global Scope

Variables in a function are assumed to be local unless marked as global:

glob = 0
def doSomething( x ):
    global glob
    glob += x
    print(glob)

--

Global variables are visible within a function but assignment creates a new local which shadows the global.

meaning_of_life = 42
def life(x):
	if x == meaning_of_life:
		return True
	else:
		return False
life(39) # false
life(42) # true

Function Arguments

Function arguments can have default values. The arguments are positional, so those with defaults must be specified last in the argument list:

def myfunc( x, y=8, z=5 ):
    print(x, y, z)

myfunc( 1, 2 ) # prints (1, 2, 5)

--

When you call a function you can specify arguments by name:

myfunc(7, z=9) # prints (7, 8, 9)

--

You can use *args and **kwargs to capture arguments without specifying them in advance:

def my_function(**kwargs):
    print str(kwargs)

my_function(a=12, b="abc") # {'a': 12, 'b': 'abc'}

Lambda Forms

You can create an anonymous function in python using lambda:

g = lambda x: x**2 # ** is the exponent operator

g(8) # 64

Map / Reduce

Map transforms a sequence into another sequence. This is equivalent to a list comprehension. Comprehensions are usually considered more 'pythonic':

map( lambda x: x**2, [1,2,3] ) # [1, 4, 9]

[ x**2 for x in [1,2,3] ]      # [1, 4, 9]

--

Reduce transforms a sequence into a single value. The lambda function takes two arguments which are the first two elements of the list, and in later iterations they are the result of the last call and the next element of the list:

f = lambda a,b: a if (a > b) else b

reduce(f, [47,11,42,102,13]) # 102

??? The reduce example is from an online course.

Modules

Use the import keyword to import functions from another module:

import random
from math import floor
floor( random.random() * 100 ) # ???

--

You create your own modules by creating a new python file (*.py), then you can import it using the filename minus the extension.

# for file mymodule.py containing function 'myfunction'
import mymodule                 # mymodule.myfunction()
import mymodule.myfunction      # mymodule.myfunction()
from mymodule import myfunction # myfunction()

Packages

A package is a directory with multiple modules and an __init__.py in it. You can import the entire package or individual modules or functions.

bash$ ls -la mypackage/
total 0
drwxr-xr-x  4 sherman  sherman  136 Jul 29 08:44 .
drwxr-xr-x  9 sherman  sherman  306 Jul 29 08:44 ..
-rw-r--r--  1 sherman  sherman    0 Jul 29 08:44 __init__.py
-rw-r--r--  1 sherman  sherman    0 Jul 29 08:44 mymodule.py

--

# we want to use function 'myfunction'
import mypackage                          # mypackage.mymodule.myfunction()
import mypackage.mymodule                 # mypackage.mymodule.myfunction()
from mypackage import mymodule            # mymodule.myfunction()
from mypackage.mymodule import myfunction # myfunction()

??? python tutorial about modules and packages

Classes

Everything in python is an object.

--

Create a class using the class keyword:

class MyClass:
    def myfunc(self, x):
        print(x)

m = MyClass()

m.myfunc(4) # 4

--

Use an __init__ function for a constructor:

class MyClass:
	def __init__(self, foo, bar):
		self.foo = foo
		self.bar = bar
	def printme(self):
		print(self.foo, self.bar)

m = MyClass(5, 7)
m.printme() # (5, 7)

Class Participation!

An exercise commonly used in job interviews is Fizz Buzz. We're going to write a Fizz Buzz solution in python using the skills you just learned.

With the numbers 1 through 100, print 'Fizz' if the number is a multiple of 3, or print 'Buzz' if the number is a multiple of 5. If the number is both a multiple of 3 and a multiple of 5, print 'Fizzbuzz'. If the number is not a multiple of either 3 or 5, print the number.

Solution 1

for x in range(100):
	n = x+1
	if n % 5 == 0 and n % 3 == 0:
		print("fizzbuzz")
	elif n % 3 == 0:
		print("fizz")
	elif n % 5 == 0:
		print("buzz")
	else:
		print(n)

Solution 2

def prn(w):
	print(w)
map( prn,
		[ ( "fizzbuzz" if x % 15 == 0 else
			( "fizz" if x % 3 == 0 else
				( "buzz" if x % 5 == 0 else
					x) ) ) for x in
				[x+1 for x in range(100)] ])

Done!

You have now completed the tutorial mission. That means you are now qualified to write python code!

Congratulations on your new skill!

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tutorial.md

tutorial.md

Sherman's One-Hour Python Tutorial

Running Python Interactively

Running a Python Script

Variables in Python

Numbers in Python

Control Flow

Loops

Exiting a Loop

Else

Sequence Types

Lists and Tuples

Slices

Dictionaries

Comprehensions

Functions

Function vs Global Scope

Function Arguments

Lambda Forms

Map / Reduce

Modules

Packages

Classes

Class Participation!

Solution 1

Solution 2

Done!

Files

tutorial.md

Latest commit

History

tutorial.md

File metadata and controls

Sherman's One-Hour Python Tutorial

Running Python Interactively

Running a Python Script

Variables in Python

Numbers in Python

Control Flow

Loops

Exiting a Loop

Else

Sequence Types

Lists and Tuples

Slices

Dictionaries

Comprehensions

Functions

Function vs Global Scope

Function Arguments

Lambda Forms

Map / Reduce

Modules

Packages

Classes

Class Participation!

Solution 1

Solution 2

Done!