Seminar 001: Python fundamentals¶

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Welcome to Jupyter!¶

Jupyter is a coding environment that combines documentation and code which is very suitable for micropublications and to explain code. It is a new fundamental in scientific scripting that aids reproduciblity. Ultimately, Jupyter can help understanding what has been done better than a plain script.

There are two types of cells:

• Code: Here the actual code will be executed
• Markdown: Explanations and any kind of text that helps understanding the code

Each code cell can be executed individually by using the play-button or the key-combo shift + enter. New cells can be added by using the plus-button (+) or the key-combo alt + enter which will execute the current cell and add a new one.

Using Python as a calculator¶

In [2]:

2 * 2

2 * 2

Out[2]:

4

Variables¶

• Variables act as "sticky notes" attached to any kind of data or result
• Most essential concept in programming
• Can be accessed later on by using the variables name
• Make sure to name your variable by its content

In [3]:

number = 2 * 2

number = 2 * 2

In [5]:

# Jupyter specific (needs to be at the end)
number

# Jupyter specific (needs to be at the end) number

Out[5]:

4

In [7]:

# Print statement
print(number)

# Print statement print(number)

4

In [8]:

# Overload the print statement
print("This is the result: ", number)

# Overload the print statement print("This is the result: ", number)

This is the result:  4

In [10]:

# Combine variables
another_number = 20

print( "Multiplying the number by 'another_number': ", number * another_number )

# Combine variables another_number = 20 print( "Multiplying the number by 'another_number': ", number * another_number )

Multiplying the number by 'another_number':  80

In [12]:

# Other operations

# Division
divided = 80 / another_number
print("Division: ", divided)

# Exponents
exponent = 80 ** 2
print("Exponent: ", exponent)

# Modulo
modulo = 80 % 2
print("Modulo: ", modulo)

# Other operations # Division divided = 80 / another_number print("Division: ", divided) # Exponents exponent = 80 ** 2 print("Exponent: ", exponent) # Modulo modulo = 80 % 2 print("Modulo: ", modulo)

Division:  4.0
Exponent:  6400
Modulo:  0

Data Types¶

• Numbers: Integers (int) and Floating Point Numbers (float)
• Strings: Collection of multiple characters (str)

In [13]:

# Integer
integer = 8

print( type(integer) )

# Integer integer = 8 print( type(integer) )

<class 'int'>

In [15]:

# Float
floating_point = 8.0

print( type(floating_point) )

# Float floating_point = 8.0 print( type(floating_point) )

<class 'float'>

In [16]:

# Strings
string = "string"

print( type(string) )

# Strings string = "string" print( type(string) )

<class 'str'>

In [17]:

# Doing operations with strings
string1 = "Hello, "
string2 = "World!"

string1 + string2

# Doing operations with strings string1 = "Hello, " string2 = "World!" string1 + string2

Out[17]:

'Hello, World!'

In [18]:

# Get sub-strings
string = "Hello, world!"

print("First letter: ", string[0])

# Get sub-strings string = "Hello, world!" print("First letter: ", string[0])

First letter:  H

In [20]:

# Get the Hello sub-string
print("This is the hello: ", string[0:5])

# Get the Hello sub-string print("This is the hello: ", string[0:5])

This is the hello:  Hello

In [22]:

# Casting of data types
floating_point = 10.6758

print("Cast the float to an integer: ", int(floating_point))

# The weird part of Python
int("10")

# Casting of data types floating_point = 10.6758 print("Cast the float to an integer: ", int(floating_point)) # The weird part of Python int("10")

Cast the float to an integer:  10

Out[22]:

10

In [23]:

# This cast does not work!
int("ten")

# This cast does not work! int("ten")

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-23-5db6fed51cc0> in <cell line: 2>()
      1 # This cast does not work!
----> 2 int("ten")

ValueError: invalid literal for int() with base 10: 'ten'

Lists¶

• Contains a sequence of arbitrary data types
• Started with [ followd by comma-separated entries and closed by ]
• Individual entries can be accessed using int indicies (my_list[0] for the first entry)
• Order is important

In [25]:

# You were in the lab and recorded a measurement
time1 = 0.0
time2 = 2.0
time3 = 5.0

# You were in the lab and recorded a measurement time1 = 0.0 time2 = 2.0 time3 = 5.0

In [27]:

# Lists are arrays of things
measurement = [ 0.0, 2.0, 5.0 ]

print(measurement)

# Lists are arrays of things measurement = [ 0.0, 2.0, 5.0 ] print(measurement)

[0.0, 2.0, 5.0]

In [29]:

print("First element: ", measurement[0])

print("First element: ", measurement[0])

First element:  0.0

In [30]:

print("First to second element: ", measurement[0:2])

print("First to second element: ", measurement[0:2])

First to second element:  [0.0, 2.0]

In [31]:

# Adding things to lists
measurement.append(10.0)

print("After appending: ", measurement)

# Adding things to lists measurement.append(10.0) print("After appending: ", measurement)

After appending:  [0.0, 2.0, 5.0, 10.0]

In [32]:

# We can add whatever we want
# It is not type specific!
measurement.append("Feierabend!")

print("After appending: ", measurement)

# We can add whatever we want # It is not type specific! measurement.append("Feierabend!") print("After appending: ", measurement)

After appending:  [0.0, 2.0, 5.0, 10.0, 'Feierabend!']

In [34]:

# Extract the last element of a list
print("The last element: ", measurement[-1])

# Extract the last element of a list print("The last element: ", measurement[-1])

The last element:  Feierabend!

Dictionaries¶

• Mapping from a key to a value
• Can be regarded as a collection of "sticky notes"
• Behaves similar to a list, but that the indices are generalized to custom indices
• Order is not important

In [37]:

# Dict is a generlization of a list
# list[1] = something
metadata = {"name": "Jan"}

print(metadata["name"])

# Dict is a generlization of a list # list[1] = something metadata = {"name": "Jan"} print(metadata["name"])

Jan

In [38]:

# Lets implement a list as a dict
fake_list = {0: 0.0, 1: 2.0}

print("First entry: ", fake_list[0])

# Lets implement a list as a dict fake_list = {0: 0.0, 1: 2.0} print("First entry: ", fake_list[0])

First entry:  0.0

For-loops¶

• Used to dynamically iterate over each entry in a list or dictionary
• Helps solving the problem of individual accessing a list
• Most used loop in Python and staright forward

Procedure¶

1. The for-loop "pulls" an entry per round from the list
2. Stored in the "variable scope" to the name you've choosen e.g. variable
3. Inside the "body" operations can be executed as usual

Example¶

for variable in my_list:
    # do something
    print(variable * 2)

In [39]:

# Access values in a list
measurement = [0.0, 2.0, 5.0, 10.0]

measurement[0]
measurement[1]
measurement[6]

# Access values in a list measurement = [0.0, 2.0, 5.0, 10.0] measurement[0] measurement[1] measurement[6]

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-39-cee00ed135c6> in <cell line: 6>()
      4 measurement[0]
      5 measurement[1]
----> 6 measurement[6]

IndexError: list index out of range

In [45]:

# Use a for loop!
for number in measurement:
  print("Current number: ", number)
  print("Do something: ", number * 10)

print("Unindented: ", number)

# Use a for loop! for number in measurement: print("Current number: ", number) print("Do something: ", number * 10) print("Unindented: ", number)

Current number:  0.0
Do something:  0.0
Current number:  2.0
Do something:  20.0
Current number:  5.0
Do something:  50.0
Current number:  10.0
Do something:  100.0
Unindented:  10.0

In [56]:

# Dont forget indenting! This does not work

for number in measurement:
print(number)

# Dont forget indenting! This does not work for number in measurement: print(number)

  File "<ipython-input-56-a79a2f00f890>", line 4
    print(number)
    ^
IndentationError: expected an indented block after 'for' statement on line 3

While-loops¶

• Behaves similar to the for-loop
• Stops whenever a specific condition is met, such as when a number is greater some value
• Does not pull entries from a list, thus it needs to be done manually
• Needs a starting condition that is evaluated at each round
• Can end up in an infinite loop
• Rarely used and should only be considered when dealing with highly dynamic tasks

Procedure¶

1. Set up a starting condition and a termination condition
2. Enter the body of the loop and perform the operation
3. At the next iteration, the condition is tested
4. If met, the loop continues. If not, the loop is terminated

Example¶

index = 0

while index <= 10:
    print(index)
    
    # Index needs to be increased manually
    index += 1

In [55]:

measurements = [0.0, 5.0, 10.0]

index = 0

while index < 3:
  number = measurements[index]

  print("At index ", index, " is number ", number)

  # Need to increment, otherwise infinite loop!
  index += 1

measurements = [0.0, 5.0, 10.0] index = 0 while index < 3: number = measurements[index] print("At index ", index, " is number ", number) # Need to increment, otherwise infinite loop! index += 1

At index  0  is number  0.0
At index  1  is number  5.0
At index  2  is number  10.0

Conditions¶

• If-else statements are very important to deal with certain situations
• Whenever a condition is met, the following code is executed
• Optionally, an else statement can be used to deal with all situations that did not match the condition

Using conditions wisely¶

• Try to use as less as possible and as much as needed
• Nesting too many ifs is a sign of bad design
• Sometimes it is helpful to deal with what you dont want (see Guard clauses)

In [ ]:

# String comparisons

names = ["Daniel", "Alan", "Marie", "Sara"]

for name in names:

  # If statements should always evaluate
  # to a boolean (True/False)
  if True:
    print(name)

# String comparisons names = ["Daniel", "Alan", "Marie", "Sara"] for name in names: # If statements should always evaluate # to a boolean (True/False) if True: print(name)

In [48]:

# String comparisons

names = ["Daniel", "Alan", "Marie", "Sara"]

for name in names:

  if name[0] == "D":
    print(name)

# String comparisons names = ["Daniel", "Alan", "Marie", "Sara"] for name in names: if name[0] == "D": print(name)

Daniel

In [51]:

# Numerical conditions

measurements = [0.0 , 5.0, 6.0, 20.0, 50.0]

for number in measurements:

  if number < 20.0:
    print(number)
  elif number == 20.0:
    print("This should be 20! ", number)
  else:
    print("Dunno what to do!", number)

# Numerical conditions measurements = [0.0 , 5.0, 6.0, 20.0, 50.0] for number in measurements: if number < 20.0: print(number) elif number == 20.0: print("This should be 20! ", number) else: print("Dunno what to do!", number)

0.0
5.0
6.0
This should be 20!  20.0
Dunno what to do! 50.0

Its all about the sauce¶

Navigating through the syntax jungle can be quite tempting and exhausting. However, most problems you will be confronted with have already been solved numerous times. Thus, knowing your sources is the utmost important skill when programming. There are multiple sites where problems are discussed and solutions are given!

Helpful sites¶

• StackOverflow - Probably the best site to search for help
• W3Schools Python - Great overview of Pythons functionality
• PythonCheatSheet - Another great overview
• Google - The most used tool in Software Development