Coding 1002 - Getting · Helpful Interactive Python Commands •dir() • Return all variable, classes, objects (collectively called “names”)available •dir(name) • Return

Shweta Palande, DevNet Developer Advocate

DEVNET-1893.c

Coding 1002 - Getting Started with Python

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Agenda

• Why Python and How to get it?

• Breaking Down Python Code

• Interacting with Python Live

• What are Libraries and How to Use Them

• Using pip to Install Libraries

• Virtual Environments

• Foundational Libraries4

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Why Python and How to get it?


http://www.python.org/about

Python is powerful... and fast;plays well with others;runs everywhere;is friendly & easy to learn;is Open.

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http://www.python.org/about


Why Python for Network Engineers

• Readable and easy to learn

• Widely available and Open Source

• Windows, Mac, Linux

• Routers & Switches

• Many relevant code samples

• Lots of training resources

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How to get Python?

• You might already have it

• http://python.org/downloads

• Package Management Tools

• Example: Homebrew for Mac

• brew install python2

• brew install python3

DevNet$ python –version

Python 2.7.12

DevNet$ python

Python 2.7.12 (default, Oct 11 2016,

05:20:59)

[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.

>>>

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http://python.org/downloads

Let’s cover some basics


Math Operations

Addition: +

Subtraction: -

Multiplication: *

Division: /

Floor Division: / /

Modulo: %

Power: * *

Numerical Operators

10

>>> 5 + 2

7

>>> 9 * 12

108

>>> 13 / 43.25

>>> 13 // 4

3

>>> 13 % 4

1

>>> 2 ** 10

1024

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Names

• Cannot start with a number [0-9]

• Cannot conflict with a language

keyword

• Can contain: [A-Za-z0-9_-]

• Recommendations for naming (variables, classes, functions, etc.) can be found in PEP8

Created with the = assignmentoperator

Can see list of variables in thecurrent scope with dir()

Variables

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>>> b = 7

>>> c = 3

>>> a = b + c

>>> a

10

= "Foo"

= "Bar"

= string_one + string_two

>>> string_one

>>> string_two

>>> new_string

>>> new_string

'FooBar'

https://www.python.org/dev/peps/pep-0008/


Conditionals

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• Syntax:

• i f expression1 :

• sta t ements …

• e l i f expression2 :

• statements…

• e l s e :

• statements…

✓ Indentation is important!

✓ 4 spaces indent recommended

✓ You can nest if statements

<

>

Comparison Operators:

Less than

Greater than

Less than or equal to

Greater than or equal to

Equal

Not Equal

Contains element

<=

>=

==

!=

i n

Combine expressions with: and, or

Negate with: not

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is less than zero")

is exactly zero")

is greater than zero")

is something else")

Conditionals | Examples

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>>> b = 5

>>> if b < 0:

... print("b

... elif b == 0:

... print("b

... elif b > 0:

... print("b

... else:

... print("b

...b is greater than zero

>>> words = "Foo Bar"

>>> if "Bar" in words:

... print("words contains 'Bar'")

... elif "Foo” in words:

... print("words contains 'Foo'")

...

words contains 'Bar'

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Modularize your code

• Defining your own Functions

• (optionally) Receive arguments

• (optionally) Return a value

Syntax:

def function_name(arg_names):

statements…

return value

...

function_name(arg_values)

Functions | Don’t Repeat Yourself

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num2

add(3, 5)

>>> def add(num1, num2):

... result = num1 +

... return result

...

>>>

>>>

8

>>> def say_hello():

... print("Hello!")

>>>

>>> say_hello()

Hello!

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Python Collections and Loops


Data Structures / Collection Data Types

16

Namet y p e ( )

Notes Example

l i s t • Ordered list of items

• Items can be different data types

• Can contain duplicate items

• Mutable (can be changed after created)

[ ‘a ’ , 1 , 18.2]

tuple • Just like a list; except:

• Immutable (cannot be changed)

( ‘a ’ , 1 , 18.2)

dictionaryd i c t

• Unordered key-value pairs

• Keys are unique; must be immutable

• Keys don’t have to be the same data type

• Values may be any data type

{“apples”: 5 ,

“pears”: 2 ,

“oranges”: 9}

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Some useful dictionary methods:

{ } . i tems( )

{ } .keys( )

{ } .va lues( )

There are many more! 😎

Dictionary Methods

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>>> d = {"a": 1, "b": 2, "c": 3}

>>> d . i t e m s ( )

d i c t _ i t e m s ( [ ( ' a ’ , 1 ) , ( ' b ’ , 2 ) , ( ' c ' , 3 ) ] )

>>> d . k e y s ( )

dict_keys([ 'a’, ' b ’ , ' c ’ ] )

>>> d . v a l u e s ( )

dict_values([1, 2 , 3] )

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https://docs.python.org/3/library/stdtypes.html#dict


Conditional Loops

while l og ica l_express ion :

statements…

Loops

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Iterative Loops

f or i n d i v i d u a l _ i tem i n

i t e r a t o r :

statements…

>>> i = 0

>>> while True:

... print(i)

... i += 1

...

0

1

2

3

4

>>> names = ["chris", "iftach", "jay"]

>>> for name in names:

... print(name)

...

chris

iftach

jay

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Breaking Down Python Code


example1.py

• Script Structure and Format

• Importing and using packages

• Variable declaration and usage

• Function creations and usage

• Basic Error Handling

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example1.py






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example1.py






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DevNet$ python example1.py

You must provide a number as a parameter to this script

Example:

python example1.py 12

DevNet$ python example1.py 5

10.0

DevNet$ python example1.py 45.4

90.8

netprog_basics/programming_fundamentals/python_part_1/example1.py

example1.py – Testing the Code

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© 2020 Cisco and/or its affiliates. All rights reserved. Cisco Public© 2018 Cisco and/or its affiliates. All rights reserved. Cisco Public

example2.py

• Reading from and writing to files

• The “with” statement

• Requesting interactive user input

• Writing to the command line


example2.py





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example2.py





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© 2020 Cisco and/or its affiliates. All rights reserved. Cisco Public© 2018 Cisco and/or its affiliates. All rights reserved. Cisco Public

DevNet$ python example2.py

What is your name? Hank

Adding new log entry

Log File Contents

-----------------

Entry logged at: 2017-07-25 23:11:16.168003 by Hank

Entry logged at: 2017-07-26 00:17:03.199335 by Hank

example2.py – Testing the Code

netprog_basics/programming_fundamentals/python_part_1/example2.py

netprog_basics/programming_fundamentals/python_part_1/example2.log

Interacting with Python Live


What is and Why use Interactive Python

• Real-time enter code and see results

• Experiment and learning

• Debug scripts that aren’t working

• Build on the output of a script

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Accessing Interactive Python

• python –i

• Basic command line

• python –i script.py

• Run a script and leave session active

• idle

• Basic interpreter client written in Python

• Virtual Environmentpython –m idlelib.idle

DevNet$ python -i

Python 3.6.2 (default, Jul 17 2017, 16:44:45)

[GCC 4.2.1 Compatible Apple LLVM 8.1.0

(clang-802.0.42)] on darwin

Type "help", "copyright", "credits" or

"license" for more information.

>>> from pprint import pprint

>>> pprint("Hello World")

'Hello World'

>>> exit()

DevNet$ python -i example1.py 21

42.0

>>> doubler(3)

6

>>> exit()

DevNet$ idle

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Helpful Interactive Python Commands

• dir()

• Return all variable, classes, objects(collectively called “names”) available

• dir(name)

• Return attributes for an object

• help(name)

• Built-in help system. Displays docs and info for object

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Demo

What are Libraries and How to Use Them


Python Libraries (Modules, Applications, etc)

• Any Python code outside of your script you want to use

• Provide some capability or data you need

• Included with statements• from library import name

• import library

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Python Libraries (Modules, Applications, etc)

• Any Python code outside of your script you want to use

• Provide some capability or data you need

• Included with statements• from library import name

• import library

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Where to get Libraries

• Write them yourself• Example: common_vars

• Included with Python itself• Example: datetime, os, sys, json

• From Python Package Index (PyPI)• Example: pip install requests

• Download and install manually• Example: ACI Toolkit from GitHub

netprog_basics/programming_fundamentals/python_part_2/common_vars.py

https://pypi.python.org/pypi

https://github.com/datacenter/acitoolkit

Using pip to Install Libraries


Pip, The Python Package Installer

• Python 2.7.6 and Python 3.4 or greater includes by default

• Integrates with PyPI for packages

• Install, Upgrade, and Uninstall packages

• “requirements.txt” in projects provide input to pip

DevNet$ pip --version

pip 9.0.1

DevNet$ pip install pyang

Collecting pyang

Downloading pyang-1.7.3-py2.py3-none-any.whl

(326kB)

100% |████████████████████████████████| 327kB

1.3MB/s

Installing collected packages: pyang

Successfully installed pyang-1.7.3

DevNet$ pip install -r requirements.txt

Collecting requests (from -r requirements.txt (line

1))

Downloading requests-2.18.2-py2.py3-none-any.whl

(88kB)

100% |████████████████████████████████| 92kB

662kB/s

.

{OUTPUT TRUNCATED}

.

Successfully installed requests-2.18.2 six-1.10.0

urllib3-1.22

netprog_basics/programming_fundamentals/python_part_2/requirements.txt

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pip Commands to Know

• Install a package• pip install package

• Upgrade a package• pip install --upgrade package

• Uninstall a package• pip uninstall package

• View all packages installed• pip freeze

• Install “requirements.txt”• pip install –r requirements.txt

DevNet$ pip freeze

asn1crypto==0.22.0

bcrypt==3.1.3

certifi==2017.4.17

cffi==1.10.0

chardet==3.0.4

cryptography==2.0

flake8==3.3.0

idna==2.5

lxml==3.8.0

mccabe==0.6.1

ncclient==0.5.3

paramiko==2.2.1

pyang==1.7.3

pyasn1==0.2.3

pycodestyle==2.3.1

pycparser==2.18

pyflakes==1.5.0

PyNaCl==1.1.2

requests==2.18.2

six==1.10.0

urllib3==1.22

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Virtual Environments


What is a Virtual Environment (venv)

• Build isolated, fully functional Python environments on a single workstation

• Virtual Environments can• Run different versions of Python

• Have different libraries installed

• Have different versions of libraries installed

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Setting Up a Virtual Environment

DevNet$ pip install virtualenv

Successfully installed virtualenv

DevNet$ virtualenv venv

New python executable in

/private/tmp/venv/bin/python2.7

Also creating executable in

/private/tmp/venv/bin/python

Installing setuptools, pip, wheel...done.

DevNet$ virtualenv venv2 --python=python3

Running virtualenv with interpreter

/usr/local/bin/python3

Using base prefix

'/usr/local/Cellar/python3/3.6.2/Frameworks/Python.fr

amework/Versions/3.6'

New python executable in

/private/tmp/venv2/bin/python3.6

Also creating executable in

/private/tmp/venv2/bin/python

Installing setuptools, pip, wheel...done.

DevNet$ source venv/bin/activate

(venv) DevNet$

* Commands on Windows Platforms slightly different

• Install the virtualenv library• pip install virtualenv

• Create the Virtual Environment• virtualenv name

• Specify Python Version• virtualenv name --python=python3

• Activate Virtual Environment• source name/bin/activate*

• Deactivate Virtual Environment• deactivate

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Installing Python Libraries in Virtual Environments

• Once activated, no different(venv) DevNet$ pip install pyang

Collecting pyang

Downloading pyang-1.7.3-py2.py3-none-any.whl

(326kB)

100% |████████████████████████████████| 327kB

1.3MB/s

Installing collected packages: pyang

Successfully installed pyang-1.7.3

(venv) DevNet$ pip install -r requirements.txt

Collecting requests (from -r requirements.txt (line

1))

Downloading requests-2.18.2-py2.py3-none-any.whl

(88kB)

100% |████████████████████████████████| 92kB

662kB/s

.

{OUTPUT TRUNCATED}

.

Successfully installed requests-2.18.2 six-1.10.0

urllib3-1.22

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Foundational Libraries


Core Python Libraries to Know and Love

• Pretty Print• from pprint import pprint

• Python Interpreter Utilities• import sys

• Operating System Interfaces• import os

• Date and Time Utilities• import datetime

* Many libraries included with core Python, see docs

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https://docs.python.org/3/library/pprint.html

https://docs.python.org/3/library/sys.html

https://docs.python.org/3/library/os.html

https://docs.python.org/3/library/datetime.html

https://docs.python.org/3/library/index.html


Prettier Printing with pprint

• Better formatting than default print() function


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Access Details about Python with sys

• Access to some details/variables concerning running state• Access command line arguments withsys.argv[]

• Access to functions that interact withthe interpreter

• Exit Python with specific error messagesys.exit("Message")

DevNet$ python -i common_vars.py "CLI Arg 1" "CLI Arg 2"

>>>

>>> import sys

>>>

>>> sys.argv[1]

'CLI Arg 1'

>>> sys.argv[2]

'CLI Arg 2'

>>>

>>> sys.exit("Error Occurred")

Error Occurred


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Interact with Files, Paths, and Environment with os

• Access and manipulate directories and files• Note: Opening files can be done withopen(filename)

• Access and Manipulate EnvironmentVariables• os.environ[var_name]


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Get your Date and Time Correct with datetime

• Create, format, and manipulate dates and times

• Time arithmetic!

• Work with timestamps and other representations


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Demo

Summing up


Review

• Why Python and How to get it?

• Breaking Down Python Code

• Script structure, imports, variables, functions & errorhandling

• Working with files and command line input

• Using lists and dictionaries, loops, and conditional statements

• Interacting with Python Live

• How to access the Python interpreter

• Using dir() and help() to explore and learn

• Understand what Python libraries are and how to use them

• Looked at Python Virtual Environments, why and how to use them

• Explored core Python libraries for displaying data, managing running scripts, and workingwith the operating system

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Documents

Coding 1002 - Getting · Helpful Interactive Python Commands •dir() • Return all variable, classes, objects (collectively called “names”)available •dir(name) • Return