Python for Systems Engineers - Part 1

python for Beginners - Part 1
”Batteries included” Programming
Marcel Neidinger
Associate Solutions Engineer, Collaboration
[email protected]
7. November 2019

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What is the plan?
• Part 1: Introduction to python3
• Part 2: Introduction to some useful python packages
• Part 3: Hands-On - Let’s code a small MVP in one hour
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About these Slides
These slides are deliberately filled with content and can serve as a reference for you.
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About these Slides
Code sample
1 p r i n t (”This i s a code snippet ”)
Exercise
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About these Slides
Code sample
1 p r i n t (”This i s a code snippet ”)
Exercise
Exercise: Some trivial proofs
Proof, for any decission problem P1 ∈
NP and P2 ∈ P that
P1 >p
P2
holds.
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Introduction

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What is a python?
• Cross plattform (Mac, Linux, Windows, Embedder)
• Easy to write - Easier to read
• ”Batteries included”
• Well integrated in the Cisco ecosystem
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What is a python?
• Cross plattform (Mac, Linux, Windows, Embedder)
• Easy to write - Easier to read
• ”Batteries included”
• Well integrated in the Cisco ecosystem
In short python is perfect for
• Large software projects
• Quick proofs of concept
• People starting their programming journey
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Who uses python?
python where we can - C++ where we must!
- Googles early development mantra
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Hello World!
1 p r i n t (” Hello world ! ”)
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https://repl.it/languages/python3
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Variables and Datatypes

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Variablen
1 a = 10
b = ” Hallo ”
3 pi = 3.14
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Datatypes
• Python is not statically typed
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Datatypes
• In English: Variables can have any datatype they want
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Datatypes
• (3 + 1) important (simple) data types
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Datatypes
String for text (and characters)
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Datatypes
int for whole numbers
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Datatypes
float for floating numbers
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Datatypes
float for floating numbers
(None for Nothing)
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Strings
1 # Strings are defined with quotation marks
a = ”This i s a s t r i n g ”
3 b = ’ This i s also a s t r i n g ’
5 # You can put them over multiple l i n e s with three ”””
c = ”””
7 This s t r i n g goes over
multiple
9 l i n e s
”””
11
# And they can be joined together
13 d = ”A s t r i n g ”
e = ”can be e a s i l y connected”
15 f = d + ” ” + e
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ints
1 # D e f i n i t i o n
a = 10
3
# Simple arithmetics with others
5 a = 10 + 5
b = a − 5
7 c = b ∗ 3
# and with themselfes
9 a = a + 3
11 # In a more pythonic way
a += 3
13 a ∗= 3
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floats
1 # D e f i n i t i o n (mind the ”.”)
a = 10.0
3
# Simple arithmetics
5 a = 10.0 + 5.0
b = a/3.0
7
# We can also add with other numbers
9 c = 10
c = i n t ( c )
11 d = 3.0 ∗ c
13 type (d)
#
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None
• What if we want to mark something as nothing?
• 0 and -1 can be ambigious
• In python we use None
a = None
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Converting data types
• python keeps track of (what it thinks) the data type of your variable is
• Often operations can only be applied to variables of the same type (Examples: +,-,...)
⇒ We need to convert between data types
1 # Simple conversion
a = 10.0
3 b = i n t (a)
5 # Strings and numbers
d = 42
7 s = ”Das i s t ein String mit der Zahl ”
s = s + ” ” + s t r (a)
9 # To check the type
type (d)
11 #
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Exercise for Data Types
Exercise: Simple Calculations and Conversions
a) Use python to calculate the following simple instructions
• 10 + 3 the result should be a float
• 10/3 the result should be a int
• 5/2 the result should be a int
• 10/4 the result should be a float
b) What is wrong with the following code fragment? Run it to find out!
1 a = 10
p r i n t (”Wir haben insgesamt ” + a + ” Probleme”)
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Solutions
Solution: Exercise for Data Types
a)
a1 = f l o a t (10 + 3)
2 # >>> 13.0
a2 = i n t (10/3)
4 # >>> 3
a3 = i n t (5/2)
6 # >>> 2
a4 = f l o a t (10/4)
8 # >>> 2.5
b)
a is variable of type int. We get a type error since only strings can be concatinated.
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Loops and Control Structures

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for-loops
• for loops allow to iterate over a set of commands n times
for i in range (0 , 10) :
2 p r i n t (”This i s the ” + s t r ( i ) + ” . i t e r a t i o n ”)
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for-loops
1 for i in range (0 , 10) :
p r i n t (”This i s the ” + s t r ( i ) + ” . i t e r a t i o n ”)
• The increment isn’t fixed to 1
for i in range (0 ,10 ,2) :
2 p r i n t (”Now we only show every second number in the range ( i=” + s t r ( i ) + ”)
”)
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for-loops
1 for i in range (0 , 10) :
p r i n t (”This i s the ” + s t r ( i ) + ” . i t e r a t i o n ”)
• The increment isn’t fixed to 1
for i in range (0 ,10 ,2) :
2 p r i n t (”Now we only show every second number in the range ( i=” + s t r ( i ) + ”)
”)
• for loops can be nested (i.e. to iterate over the pixels in an image)
for x in range (0 ,10) :
2 for y in range (0 ,10) :
pr i nt (” Searching in coordinate (” + s t r (x) + ” , ” + s t r (y) + ”)”)
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Exercise for for-loops
Exercise: Calculating multiplication tables
In this Exercise we will calculate multiplication tables like this
1 M u l t i p l i c a t i o n table for 2
2 x 1 = 2
3 2 x 2 = 4
2 x 3 = 6
5 2 x 4 = 8
[ . . . ]
7 2 x 10 = 20
Tip Use two nested for loops to calculate all multiplication tables from 1 to 10
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Exercise for for-loops (contd.)
Solution: Calculating multiplication tables
1 for num in range (1 , 11) :
p r i nt (” M u l t i p l i c a t i o n table for ” + s t r (num) )
3 for m u l t i p l i e r in range (1 , 11) :
res = num ∗ m u l t i p l i e r
5 pr i nt ( s t r (num) + ” x ” + s t r ( m u l t i p l i e r ) + ” = ” + s t r ( res ) )
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Control structures - the if clause
• Using if-clauses we can change the program flow based on variable values
• if-clauses need a comparisson
• Instructions caried out when a comparisson is true are called a branch
1 a = 10
i f a == 10:
3 # This i s a branch
p r i n t (”a has the value ” + s t r (a) )
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Control structures - the if clause
• Using if-clauses we can change the program flow based on variable values
• if-clauses need a comparisson
• Instructions caried out when a comparisson is true are called a branch
1 a = 10
i f a == 10:
3 # This i s a branch
p r i n t (”a has the value ” + s t r (a) )
• You can add other possible outcomes
2 i f i%2 == 0:
pr i nt ( s t r ( i ) + ” can be divided by two without a remainder ”)
4 e l i f i%5 == 0:
pr i nt ( s t r ( i ) + ” can be divided by f i v e without a remainder ”)
6 e l s e :
pr i nt ( s t r ( i ) + ” can not be divided by e i t h e r f i v e or two without a
remainder ”)
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Exercise for if-clauses
Exercise: FizzBuzz - A programming interviewers best friend
FizzBuzz is a simple test done in programming interviews around the world.
Write a program that prints the numbers from 1 to (including) 20
• For multiples of three print Fizz instead of the number
• For multiples of five print Buzz instead of the number
• For multiples of both three and five print FizzBuzz instead of the number
Tip a is a multiple of b if a%b == 0
Tip You can chain multiple conditions with and
1 i f age > 18 and age < 50:
p r i nt (”You are young enough”)
3
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Exercise for if-clauses (contd.)
Solution: FizzBuzz - A programming interviewers best friend
for num in range (1 , 21) :
2 i f num % 3 == 0 and num % 5 == 0:
pr i n t ( ’ FizzBuzz ’ )
4 e l i f num % 3 == 0:
pr i n t ( ’ Fizz ’ )
6 e l i f num % 5 == 0:
pr i n t ( ’Buzz ’ )
8 e l s e :
pr i n t ( s t r (num) )
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Functions

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Functions
• Often we need to re-use parts of our code
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Functions
• Copy & Paste is never the correct answer
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Functions
• Copy & Paste is never the correct answer
• Example: FizzBuzz from the last Exercise
1 def fizzbuzz () :
for num in range (1 , 21) :
3 i f num % 3 == 0 and num % 5 == 0:
p r i n t ( ’ FizzBuzz ’ )
5 e l i f num % 3 == 0:
p r i n t ( ’ Fizz ’ )
7 e l i f num % 5 == 0:
p r i n t ( ’Buzz ’ )
9 e l s e :
p r i n t (num)
11 fizzbuzz ()
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Functions (contd.)
• Functions can have arguments
1 def fizzbuzz (n) :
for num in range (1 , n) :
3 . . .
• Functions can return stuff
1 def fizzbuzz_generator (num) :
i f num % 3 == 0 and num % 5 == 0:
3 return ”FizzBuzz”
e l i f num % 3 == 0:
5 return ” Fizz ”
e l i f num % 5 == 0:
7 return ”Buzz”
e l s e :
9 return s t r (num)
11 p r i nt ( fizzbuzz_generator ( i ) )
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Exercise for Functions
Exercise: Maximum of Three Numbers
Write a function that returns the maximum of three numbers x,y and z.
Tip Start with a function that returns the maximum of two numbers and then use that function
inside the function that calculates the maximum of three.
Tip The Signature should be
1 def max_of_two(x , y) :
. . .
3
def max_of_three (x , y , z ) :
5 . . .
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Exercise for Functions (contd.)
Solution: Maximum of Three Numbers
1 def max_of_two(x , y) :
i f x > y :
3 return x
e l s e :
5 return y
7 def max_of_three (x , y , z ) :
return max_of_two(x , max_of_two(y , z ) )
9 p r i n t ( s t r ( max_of_three (9 , 4 , 3) ) )
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Advanced Data structures

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Why do we need even more structures? Chaos is awesome!
• We already have variables. What else could we need?
• What if we don’t know the number of variables?
• What if we have a list of variables?
• What if a function should return more then one variable?
⇒ We need mooooore
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Lists
• lists save lists of objects
1 l = [ ]
• We can add objects of different types to lists
1 l = [ ”One” , 2 , ” three ” , None ]
p r i n t ( l )
3 # [ ’One ’ , 2 , ’ three ’ , None ]
• We can access list items by their index (Attention Index starts at 0)
1 l = [ ”one” , ”two” , ” three ” ]
p r i n t ( l [ 0 ] )
3 # one
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Lists (contd.)
• We can append, manipulate and delete items from a list
1 l = [ ”one” , ”two” , ” three ” ]
# Remove the f i r s t element with VALUE one
3 l . remove (”one”)
# [” two” , ” three ”]
5 # Remove the element at index 1
del l [ 1 ]
7 # [” two ”]
# Append to the end of the l i s t
9 l . append (” four ”)
# [” two” , ” four ”]
11 # Append at a s p e c i f i c posit ion
l . i n s e r t (0 , ” s i x ”)
13 # [” s i x ” , ”two” , ” four ”]
# L i s t s can contain duplicates (more on t h i s l a t e r )
15 l = [1 , 2 , 3]
l . append (1)
17 # [1 ,2 ,3 ,1]
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for-loops revisited
• You have been using lists all along
1 # This function returns a l i s t
range (0 , 10)
3
• We can iterate over a list using range and the number of elements
l = [ ”one” , ”two” , ” three ” , ” four ” ]
2 for i in range (0 , len ( l ) ) :
p r i n t ( l [ i ] )
4
• You can also loop over lists with in (for-each syntax)
1 l = [ ”one” , ”two” , ” three ” ]
for element in l :
3 p r i n t (”− ” + s t r ( element ) )
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Where is waldo with lists
• You can easily check if an item is in a list
1 l = [ ”one” , ”two” , ” three ” ]
i f ”one” in l :
3 p r i n t (”One i s in the l i s t ”)
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Exercises for Lists
Exercise: Set theory for lists
In set theory we have the intersection(all elements in both lists) and the union(all unique
elements in both lists combined).
Example:
l1 = [1, 2, 3]
l2 = [2, 3, 5]
then
l1 ∩ l2 = [2, 3]
Write a function that calculate the intersection of lists of integers. Test with the following
two lists:
1 l1 = [1 , 2 , 3]
l2 = [2 ,4 ,5]
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Exercises for Lists
Solution: Set theory for lists
l1 = [1 , 2 , 3]
2 l2 = [2 , 4 , 5]
def i n t e r s e c t i o n ( l1 , l2 ) :
4 s = [ ]
for e in l1 :
6 i f e in l2 :
s . append ( e )
8 return s
10 p r i n t (”The i n t e r s e c t i o n i s ” + s t r ( i n t e r s e c t i o n ( l1 , l2 ) ) )
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Dictionaries
• Typical problem: Saving key-value pairs
• Settings
• User data (name ⇒ Marcel, age ⇒ 22)
• Naiive idea: Two lists with coresponding indices
keys = [ ]
2 values = [ ]
4 def get ( key , keys , values ) :
for i in range (0 , len ( keys ) ) :
6 i f key == keys [ i ] :
return values [ i ]
Just because it works that doesn’t mean that it is good
• Problems?
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Dictionaries
• Typical problem: Saving key-value pairs
• Settings
• User data (name ⇒ Marcel, age ⇒ 22)
• Naiive idea: Two lists with coresponding indices
1 keys = [ ]
values = [ ]
3
def get ( key , keys , values ) :
5 for i in range (0 , len ( keys ) ) :
i f key == keys [ i ] :
7 return values [ i ]
Just because it works that doesn’t mean that it is good
• Problems?
• python comes with a build-in data type for this
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Dictionaries (contd.)
• Initialization
1 d = { ’ one ’ : ’ h e l l o ’ , ’two ’ : ’ world ’ , ’ three ’ : ’ ! ’}
p r i n t (d [ ’ one ’ ] )
3 p r i n t (d [ ’two ’ ] )
p r i n t (d [ ’ three ’ ] )
• We often want to access all elements in a dictionary
for key , value in d . items () :
2 p r i nt ( s t r ( key ) + ” =
> ” + s t r ( value ) )
# one =
> h e l l o
4 # two =
> world
# three =
> !
• Sometimes we want just the keys (or the values)
1 d = { ’ one ’ : ’ h e l l o ’ , ’two ’ : ’ world ’ , ’ three ’ : ’ ! ’}
for key in d . keys () :
3 i f key == ”one” :
p r i n t (”Value i s ” + s t r (d [ key ] ) )
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Exercises for Dictionaries
Exercise: Counting Words and Characters in a text
Copy the text from the webex teams chat and count …
a) The number of occurences for each letter
b) The number of occurences for each word (ignoring upper and lowercase)
Tip Use the split to split a space-seperated string into its words.
a = ”This i s a text ”
2 s p l i t = a . s p l i t (” ”)
p r i n t ( s p l i t )
4 # [ ’ This ’ , ’ i s ’ , ’a ’ , ’ text ’]
Tip You can iterate over the letters in a string like it is a list
a = ” spaces ”
2 for char in a :
p r i nt (” l e t t e r : ” + s t r ( char ) )
4
Tip Call the lower() method to convert a string into all lowercase. 35

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Exercises for Dictionaries (contd.)
Solution: Counting Words and Characters in a text
a) Number of letters
1 long_string = ” . . . ”
count = {}
3 for l e t t e r in long_string :
i f l e t t e r in count . keys () :
5 count [ l e t t e r ] += 1
e l s e :
7 count [ l e t t e r ] = 1
p r i n t (” I found the following l e t t e r s ”)
9 p r i n t ( s t r ( count ) )
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Exercises for Dictionaries (contd.)
Solution: Counting Words and Characters in a text
b) Number of words
1 word_cnt = {}
words = [ ]
3 for w in long_string . s p l i t (” ”) :
words . append ( s t r (w) . lower () )
5
for word in words :
7 i f word in word_cnt . keys () :
word_cnt [ word ] += 1
9 e l s e :
word_cnt [ word ] = 1
11 p r i n t ( s t r ( word_cnt ) )
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Input/Output

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Reading files
• Reading files is as easy as eating a pie in python.
1 f i l e _ i n = open (” te s t . txt ” , ” r ”)
for l i n e in f i l e _ i n . r e a d l i n e s () :
3 pr i nt (” Line : ” + s t r ( l i n e ) )
f i l e _ i n . close ()
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Writing files
• Writing files is as easy as eating a pie in python.
l i n e s = [ ” t h i s i s the f i r s t l i n e ” , ”This i s the second l i n e ” ]
2 file_out = open (” test_out . txt ” , ”w”)
for l i n e in l i n e s :
4 file_out . write ( l i n e + ”\n”)
file_out . close ()
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Exercise for File I/O
Exercise: A Simple Copy Software
Write a simple script that copies a source file into the destination file. You only need to
handle text files and can read the content.
Tip Start with this code snippet
1 IN_FILE_NAME = ”. txt ”
OUT_FILE_NAME = ”. txt ”
3
f i l e _ i n = open (IN_FILE_NAME, ” r ”)
5 file_out = open (OUT_FILE_NAME, ”w”)
7 # Your code here
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Übungen zu File I/O
Solution: A Simple Copy Software
1 IN_FILE_NAME = ”. txt ”
OUT_FILE_NAME = ”. txt ”
3
f i l e _ i n = open (IN_FILE_NAME, ” r ”)
5 file_out = open (OUT_FILE_NAME, ”w”)
7 for l in f i l e _ i n . r e a d l i n e s () :
out . write ( l )
9
f i l e _ i n . close ()
11 file_out . close ()
p r i n t (”Copied ” + s t r (IN_FILE_NAME) + ” to ” + s t r (OUT_FILE_NAME) )
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The standard library

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Batteries included?
• pythones comes with a lot of functionality included
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Build-in functions
• Don’t reinvent the wheel!
• → Your programming skills are (very probably) worse then those who implemented the
standard libary
• Batteries included programming
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random - Accessing the randomness
import random
• Get a random integer in a range
1 rand_int = random . randint (0 , 100)
• Random element from a list
1 l = [ ”one” , ”two” , ” three ” ]
rand_elem = random . choice ( l )
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Projects
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A (few) Words of Wisdom
To think that you can become good at programming by watching a lecture or reading
a book is like thinking that you can learn to drive by watching Formula One.
- Unknown
• This course is about the basics. Now you have to program.
Fail early - Fail often
- Facebook Engineering Mantra
• Failing (and the associated searching of errors) is part of the programming experience!
Programmieren ist wie Puzzle spielen nur das alle 5 Minuten ein wild gewordener Affe
die Stücke wieder durcheinander bringt.
- Unknown
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Projects - Introduction
• Following are three proposed programming projects you can work on
• Projects one and two are slightly more complex (and math-heavy)
• If you have your own idea: go with it
• Feel free to collaborate on the projects
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Projects - Introduction
• Following are three proposed programming projects you can work on
• Projects one and two are slightly more complex (and math-heavy)
• If you have your own idea: go with it
• Feel free to collaborate on the projects
Walk-In Clinic
Monday to Friday 11:15 - 12:00, 4th floor
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Project 1 - Numerical Integration with Polynoms
The goal of this project is to calculate the area under a polynom in the interval [0, 10]. The
polynom is defined in a text file and is passed on to the program at runtime.
$ python3 integrate . py polynom . txt
As a reminder, polynoms can be defined by their coefficients as
p(x) =
n
∑
i=0
αi
xi
and for integration the trapez rule defines, for a interval [c, d] and a function f
d
∫
c
f dx ≈ QT
[c,d]
(f) =
(d − c)
2
[f(c) + f(d)]
Bonus: For better precission you can use the summed trapez rule that uses N subintervals
(xi, xi+1)∀i = 1, . . . , N + 1 mit xi = c + (i − 1)/h with fixed interval width h = (d−c)
N
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Project 2 - Least-Square Fitter
The goal of this project is to calculate a best-fit line between datapoints. The datapoints(x,y
pairs) should be read from a text file and the program should return the fit-parameters.
As a reminder: We look at the over-determined system of linear equations (in matrix notation)
n
∑
j=1
Xijβj = yi , (i = 1, . . . , m)
with m linear equations and n coefficients. In matrix notation: Xβ = y. Since the system is
over-determined there is no gurantee that a perfect solution exists. We therefor look at the
optimization problem
ˆ
β = arg min
β
S(β) with cost function S(β) = ||y − Xβ||2
Some calculations then yield
ˆ
β = (XTX)−1XTy
as a solution. Tip XTX is p.s.d. ⇒ Use Cholesky decomposition(C = LTL) that defines C−1 = (L)−1(LT)−1.
L is a triangular matrix. Inversion follows trivially from back-substitution. 48

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Project 3 - Encrypting Text
The goal of this project is to develop a simple encryption/decryption software based on the
Caesar algorithm. The path to a single text file or a directory containing text files is passed to
the program as an argument. The content of each of these files should then be encrypted and
put into a .crypt.txt file. For encryption use the shift constant K that can be passed by the user.
1 $ python3 caesar . py 3 docs/
As a reminder: The Caesar algorithm uses a constant shift constant K. For a text each letter is
shifted by that constant in the alphabet. As an example for K = 3 A ⇒ D , B ⇒ E , . . ..
Bonus: Write another program that can decrypt the files again.
Bonus: The caesar encryption is not cryptographically sound. If you have access to a sufficiently
long text you can find the constant K by counting the occurences of each letter. The most
common letter in the English language is the e. Write a program that ”cracks”your encryption.
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Questions? Nothing working?
[email protected]
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Additional advanced data structures

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Tuples
• tuples are immutable lists
• In English: Once created the elements in a tuple can not be altered
1 t = (1 , 2 , 3)
t = ( ’ Marcel ’ , 22)
• Accessing an element
p r i n t (”The element i s ” + s t r ( t [ 0 ] ) )
2
• Tuples with one element still need a comma
1 t = (1 ,)
• We can’t add/remove/delete parts of a tuple ⇒ create a new one with parts of others
1 t1 = (1 ,)
t2 = (3 , 4 , 5)
3 t = t1 + t2 [ 1 : 3 ]

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Sets
• sets are unorderd, unique lists
• In English: The order in which elements are added is ignored
• In English: Every value occures only once in a set
• Operations are same as on lists
A = {1 , 2 , 3}
2 A. add (4)
# Discard w i l l ignore non−e x i s t i n g elements . remove w i l l throw an e r r o r
4 A. discard (5)
A. remove (5)
• Basic set operations are also supported
1 A = {1 , 2 , 3}
B = {3 , 4 , 5}
3 A. union (B)
A. i n t e r s e c t i o n (B)
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Additional parts of the standard library

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os - Access to the Operating System
import os
• Create a folder
1 os . mkdir (” testordner ”)
• Rename a folder or a file
1 os . rename(” testordner ” , ” testordner2 ”)
• Join paths
1 path = os . path . j o i n (”/home/marcel” , ”Downloads”)
• Delete files and folders
1 os . remove (” datei . txt ”) # Delete f i l e
os . rmdir (” testordner /”) # Delete f o l d e r
3 os . rmdirs (” testornder /”) # Delete f o l d e r r e c u r s i v l y

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datetime - Calculating dates and times
1 import datetime
• Get current time
1 now = datetime . datetime . now()
• Specify a future time
1 # year , month , day , hour , minute , seconds , microseconds
l a t e r = datetime . datetime (2019 , 2 , 21 , 10 , 8 , 22 , 0)
• Calculating differences
delta = l a t e r − now
2 p r i n t (”The d i f f e r e n c e in seconds i s ” + s t r ( delta . seconds ) )

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sys - Accessing system information
import sys
• Accessing command line parameters (i.e. when calling python3 test.py parameter1
parameter2)
1 p r i n t (”Name of the s c r i p t : ” + s t r ( sys . argv [ 0 ] ) )
# In the example : te s t . py
3 p r i n t (” F i r s t parameter : ” + s t r ( sys . argv [ 1 ] ) )
# In the example : parameter1

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math - Basic mathematics
import math
• Constants like π, e, ...
1 PI = math . pi
• Triangular functions (note that all values are in radians)
1 math . sin (0.5)
math . cos (0.5)
• Convert angle from radians to degrees and vice-versa
math . degrees (0.5) # radians to degrees
2 math . radians (290) # degrees to radians
• Math functions
math . pow(x , y) # x^y
2 math . sqrt (x) # square root of x

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Useful resources

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Links
• The python3 standard library:
https://docs.python.org/3/library/
• Installing python for development:
Mac: https://docs.python-guide.org/starting/install3/osx/
Windows: https://docs.python-guide.org/starting/install3/win/
Linux: https://docs.python-guide.org/starting/install3/linux/
• The core concepts of python:
https://www.python-course.eu/python3_history_and_philosophy.php
• Setup the atom editor for python development:
https://hackernoon.com/
setting-up-a-python-development-environment-in-atom-466d7f48e297

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Python for Systems Engineers - Part 1

Python for Systems Engineers - Part 1

Marcel Neidinger

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