Python 3 Tutorials and Notes

2to3.py

2to3 Python program that reads Python 2 source code and applies a series of fixers to transform it into valid Python 3 code.

example1.py file content of Python 2 code at "C:\Python3\" folder:
def greet(name):
    print "Hello, {0}!".format(name)
print "What's your name?"
name = raw_input()
greet(name)

C:\Python3> python example1.py
  File "example1.py", line 2
    print "Hello, {0}!".format(name)
                      ^
SyntaxError: invalid syntax

C:\Python3> python "C:\Python3\Tools\Scripts\2to3.py" -w example1.py

C:\Python3> python example1.py
What's your name?
Linux Mint Mate
Hello, Linux Mint Mate!

A backup of the original Python version 2 file is made and the new file name is called "example1.py.bak".

2to3.py writes the needed modification right back to the source file.
Writing the changes back is enabled with the -w flag.

example1.py file content of Python 3 code:
def greet(name):
    print("Hello, {0}!".format(name)) #changed
print("What's your name?") #changed
name = input() #changed
greet(name)

Note that, this is not a fool-proof conversion though, and you'll likely need to do some bug-fixing here and there, but it may well be quicker.

Program IDLE Python 3 GUI 64 bit: Shell and File Editor

Start Menu > Python 3 > IDLE (Python 3 GUI - 64 bit)

Python 3 Shell window:
>>> import os
>>> os.getcwd()
'C:\\Python3'
>>>

Python 3 File Editor:

Shell Menu > File > New File Ctrl+N

On the file editor window > press Ctrl+S

Save the file name as time_sleep_eg1.py

Click the Save button

time_sleep_eg1.py file content:
###
import time

repeat = "y"
sentence = []

def sentenceinput():
    global sentence, repeat
    sentence_length = input("Sentence length: ")
    sentence_length = int(sentence_length)
    iteration = 0
    while iteration < sentence_length:
        print("Sentence word ", iteration+1, "? ", sep="", end="")
        sentence.append(input())
        iteration += 1
    print(*sentence)
    repeat = input("Repeat? (y/n): ")

print("Let us create one sentence only.")
while True:
    time.sleep(1)
    if repeat == "y":
        sentenceinput()
    else:
        print("Goodbye!")
        break

###

On the file editor window, press F5 to run the module file

Shell output:
###
>>> ================================ RESTART ================================
>>>
Let us create one sentence only.
Sentence length: 3
Sentence word 1? word1
Sentence word 2? word2
Sentence word 3? word3
word1 word2 word3
Repeat? (y/n): y
Sentence length: 2
Sentence word 1? word4
Sentence word 2? word5
word1 word2 word3 word4 word5
Repeat? (y/n): n
Goodbye!
>>>
###

Edit your .py script and press F5 as needed to re-run the script.

Write your Own Function
Code:
def frange(start, stop, step):
    i = start
    # use floating point inequality check formula
    # avoid "while i < stop:"
    while (abs(stop-i) > 1e-6):
        #print("{0:.1f}".format(i))
        yield i
        i += step

#frange(0.0, 0.9, 0.1)

for i in frange(0.0, 0.9, 0.1):
    #print("{0:.1f}".format(i), end=" ")
    print(round(i,1), end=' ')
print()  #print new line

Output:
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Float point inequality test formula:

abs(b - a) > epsilon-error-tolerance

Control structure: while loop

Code:
*listofthings, loops = range(6)
print(listofthings)
print(loops)
loop = 0
while loop < loops:
    listofthings[loop] = "thing"+str(loop)
    loop += 1
print(listofthings)

Output:

[0, 1, 2, 3, 4]                                                                                     

5                                                                                                   

['thing0', 'thing1', 'thing2', 'thing3', 'thing4'] # listofthings is still a list data structure but the list values modified using array syntax

Function range()
In Python 2 range() produced a list.

In Python 3, the range() function got its own type. In basic terms, if you want to use range() in a for loop, then you're good to go. However you can't use it purely as a list object. For example, you cannot slice a range type.

sh-4.2# python                                                                                      

Python 2.7.5                                       

>>> range(5)                                                                                        

[0, 1, 2, 3, 4]                                                                                     

>>> type(range(5))                                                                                  

<type 'list'>                                                                                       

>>> exit                                                                                            

Use exit() or Ctrl-D (i.e. EOF) to exit                                                             

>>> exit()
sh-4.2#                                                                                        

sh-4.2# python3                                                                                     

Python 3.3.2                            

>>> range(5)                                                                                        

range(0, 5)                                                                                         

>>> type(range(5))                                                                                  

<class 'range'>                                                                                     

>>> exit()                                                                                          

sh-4.2#

Note that in Python 3, you can still produce a list by passing the generator returned to the list() function as follows:
list1 = list(range(5))
print(list1)

# output is [0, 1, 2, 3, 4] 

Human Input syntax

input([prompt])

Python 2 statement uses raw_input to get your input data.

Code:
name = input("What's your OS name? ")
print("My OS name is", name)
Output:
What's your OS name? Linux Mint Mate                                                               
My OS name is Linux Mint Mate  

Code:
a = input("Enter first number: ")
b = input("Enter second number: ")
a, *numbers, b = range(int(a),int(b)+1,1)
print("The first number is ", a, ",",
"\nthe second number is ", b, " and",
"\nthe numbers between them are ", numbers, ".", sep="")

Output:

Enter first number: 5                                                                               

Enter second number: 10                                                                             

The first number is 5,                                                                              

the second number is 10 and                                                                         

the numbers between them are [6, 7, 8, 9].

Data Structure Dictionary

Code:

dict1 = { "1":"word1", "2":"word2", "3":"word3" }
for key in dict1:
    print(key, ":", dict1[key])

Output:

1 : word1                                                                                           

2 : word2                                                                                           

3 : word3 

Data Structure List

list1=["word1", "word2", "word3", "."]
print(list1)
print(len(list1))
print(*list1)
del list1[:]
print(list1)

#Output:

# ['word1', 'word2', 'word3', '.']
# 4                                                                  

# word1 word2 word3 .                                                                                 

# []

The asterisk allows for each item to be called individually rather than the list.

print syntax

print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)

sep --- parts separator, by default it is a space, you can change it to any compatible character that Python allows - this can be used to make the line shorter or include mathematical operator such as >.
print("Hello, World!")

print("1","2","3", sep="<")

# output is 1<2<3

num = 4
print("The number num is ", num, " or 2*2=", 2*2, sep="")

# output is:

# python3 main.py                                                                             

# The number num is 4 or 2*2=4 

end --- end of the line, to leave a space rather than starting a new line, you can use print(x, end=' '). By default, it creates a line break.

print("line1","line1;",end=" ")
print("line1 also")

# output is line1 line1; line1 also 

file=sys.stdout --- you construct the string as you would normally to print out anything and then append the file option

From Python 2 code:
print >>sys.stderr, "an error"
print "no error"

To Python 3 code:
import sys
print("an error", file=sys.stderr)
print("fatal error", file=sys.stderr)
print("no error")

flush --- if True will force the print stream to flush after execution

Python Comment Symbol

Python comment symbol is #

# single line comment

Python Abbreviations

PEP --- Python Enhancement Proposal

PSF --- The Python Software Foundation is an organization devoted to advancing open source technology related to the Python programming language

PyPI --- Python Package Index

Python 3 Information

Python 3 was to make changes to the language primarily to remove the warts that had grown in the preceding versions.

Python 3 was not to be a complete redesign(revolution), but instead an evolution of the language.

Maintaining "full" backward compatibility, unnecessary syntax, unnecessary semantic acceptable with Python 2 were explicitly off-the-table.

Python 2 code can be translated fairly easily to Python 3, sometimes entirely mechanically by such tools as 2to3. There is also non-trivial subset of the language that will run without modification on both versions 2 and 3.

The end of life for Python 2.7 has been announced.
Python 2.7 itself is the last version of the Python 2.x line with firm plans to not release a Python 2.8.

Python 2.7 will stop receiving bug fixes and other updates in 2020.

Get your code ported over to Python 3.

Python 4 that is not going to happen any time soon so you should be fine to safely port to version 3 without worrying about a massive jump in the future.

Internet Resources

Download Python 3
https://www.python.org/downloads/

Online Python 2 Interpreter
http://repl.it/languages/Python

Online Python 3 Interpreter
http://www.tutorialspoint.com/execute_python3_online.php

Stack Overflow, this website operates on a reward system for correct answers, so people are always willing to pitch in and help out others
http://stackoverflow.com/

Linux Questions
http://www.linuxquestions.org/

What is new in Python 3.0
https://docs.python.org/3/whatsnew/3.0.html