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Introduction to Python, Part 2

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0. Loops and Conditionals

This lesson is the second in a series of workshops that provide an introduction to programming using Python.

This lesson builds on the foundation of lesson 1 and covers: - lists and dictionaries - methods - loops - booleans - conditional expressions

1. Data structures

Often, when you're working with values in science, you're not just working with x = 1 and y = 2, but instead with large amounts of data. Python offers a variety of data structures that aggregate data, including:

  • lists: ordered arrays of values.
  • dict or dictionaries: values ordered with key-value pairs.
  • sets: like lists, but each value can only occur once.
  • tuples: like lists, but you can't modify the values.

We'll first talk about list and dict structures, as they're the most common data structures.

1.1 Lists

You can create a list in Python using bracket notation, such as below.

lst = [1, 2, 3, 4, 5]

To access a specific value in the list, such as 2, you use the index, or the position of the value in the list, in bracket notation, as below.

A key thing to remember in Python is that lists are indexed starting with the index [0]. So, if you wanted to get the value 2 from the list above, you would use the index [1].


Lists are powerful because you can do many different operations with them using built-in functionality. For example, you can slice lists to get only specific elements using the colon operator :.

To get the first through third elements of a list, you would use the following notation:

[1, 2, 3]

Note that, in the above example, the elements are extracted with a 'left-included, right-excluded' notation - the fourth element in the list, 4, at index 3, is not included.

You can slice and index lists in some pretty interesting ways, such as in the examples below:

lst = [0, 1, 2, 3, 4, 5, 6, 7, 8]

# The below expression is equivalent to lst[0:3].
# It gives you everything from the start of the list up to the ending address.

# The below expression gives you the last element in the list.

# The below expression gives you the last two elements in the list.
[0, 1, 2]
[7, 8]

Note that accessing a slice of a list returns a list, whereas accessing a single element returns the value itself.

1.2 List Methods

Many data types in Python have special functions built-in to their data type. These special functions are called methods and are accessed using a dot operator, or period (.). For example, you can add new elements to a list using the .append() method.

fruit_trees = ["apple", "orange", "peach", "pear", "cherry"]

new_fruit = "coconut"


['apple', 'orange', 'peach', 'pear', 'cherry']
['apple', 'orange', 'peach', 'pear', 'cherry', 'coconut']

There are many different list methods. You can take a look at some of them at the Python documentation page.

1.3 List indexing tricks

You can actually apply list indexing to certain other data types, such as str.
str objects can be treated like lists, where each character is an element of the list. For example:

fruit = "pineapples"


1.4 Dictionaries

Dictionaries are another data structure in Python, which allows for assigning a key to each value in the structure. Dictionaries are created using specific curly brace {} notation as below:

# Dictionaries are instantiated using curly braces.
# Each key-value pair in the dictionary is assigned using the notation:
##  'key': 'value'
# Entries are separated using commas.

favorite_fruits = {"Anabelle": "peach", "Becky": "pear", "Cole": "coconut"}

{'Anabelle': 'peach', 'Becky': 'pear', 'Cole': 'coconut'}

To access the value in a dictionary, you index by the key.
This returns the value assigned to that key.


Sometimes you might want just the keys or the values in your dictionary.
You can access those with the .keys() and .values() methods.

villagers = favorite_fruits.keys()

fruits = favorite_fruits.values()
dict_keys(['Anabelle', 'Becky', 'Cole'])
dict_values(['peach', 'pear', 'coconut'])

These methods return special types of data structures: dict_keys and dict_values.
It's important to note that these are not lists, and operations or methods that you can normally perform on lists might not work on these data structures.

However, you can always use typecasting to coerce dict_keys and dict_values structures into list format.

['Anabelle', 'Becky', 'Cole']

P1. Practice

Let's explore some of the methods available for different data types.
Let's also practice using search engines to look for our functionality of interest.

Practice 1

What do each of the methods below do?
a) Try each method individually and determine what it does.
b) Write down the type and expected value for each output variable at the end of this script.

Practice 1 Answer
a) What each method does:
.split(): breaks a string, returning a list using a separator, in this case a space (' ')
.upper(): capitalizes all alphabetic characters in the string
.find(): returns the int address where the first instance of the provided pattern ('dog') appears
.partition(): like split, only returns a tuple with the string before, the query ('fox'), and the string after
.remove(): removes the given element from a list in-place, returning None
.copy(): creates a copy of the list, returning the copy
.reverse(): reverses the order of the list in-place, returning None

b) Expected values at the end of the script:
output_1 = ['The', 'quick', 'brown', 'fox', 'jumps', 'over', 'the', 'dog.']; type: list
output_3 = 40; type: int
output_4 = ('The quick brown ', 'fox', ' jumps over the lazy dog.'); type: tuple
output_5 = None; type: NoneType
output_6 = ['dog.', 'the', 'over', 'jumps', 'fox', 'brown', 'quick', 'The']; type: list
output_7 = None; type: NoneType
my_sentence = "The quick brown fox jumps over the lazy dog."

output_1 = my_sentence.split(' ')
output_2 = my_sentence.upper()
output_3 = my_sentence.find('dog')
output_4 = my_sentence.partition('fox')

output_5 = output_1.remove('lazy')
output_6 = output_1.copy()
output_7 = output_6.reverse()


A very helpful method for str datatypes is the .format() method.
This method allows you to replace a bracketed space or variable name into a string, as below.

example_1 = "The quick {} fox jumps over the {} dog.".format("red", "happy")
example_2 = "The quick {0} fox jumps over the {1} dog.".format("arctic", "sleepy")
example_3 = "The quick {foxtype} fox jumps over the {dogtype} dog.".format(foxtype = "silver", dogtype = "lucky")

print(example_1, example_2, example_3, sep = '\n')
The quick red fox jumps over the happy dog.
The quick arctic fox jumps over the sleepy dog.
The quick silver fox jumps over the lucky dog.

Practice 2

a) How can you use .format() to adjust the following text so that it displays the price in dollar notation (\$1.00)?"
b) How could you convert this to instead display things in yen format (¥100) where one dollar = 100 yen?

Practice 2 Answer
a) print("Apples cost ${price:.2f} each.".format(price = cost))
b) print("Apples cost ¥{price} each.".format(price = cost * 100))
cost = 1

print("Apples cost {price} each.".format(price = cost))
Apples cost 1 each.

2. Loops

Often when working with scientific data, you might want to apply the same set of transformations to many different samples. You can repeat a process in Python and other programming languages using loops.

2.1 for loops

A basic example of this approach is the for loop.

Let's say you had a bunch of different samples with different ID numbers, and you wanted to create a sample name as a string for each. You could take the following approach and manually do that transformation:

sample_ids = [100, 231, 572]

sample_1 = "sample_" + str(sample_ids[0])
sample_2 = "sample_" + str(sample_ids[1])
sample_3 = "sample_" + str(sample_ids[2])

print(sample_1, sample_2, sample_3)
sample_100 sample_231 sample_572

A faster way to do this would be to use a for loop.

The for loop will repeat for every element in a list, as below:

for i in [0, 1, 2]:

You can combine a for loop with your data structure to be able to iterate through your list, as follows:

sample_ids = [100, 231, 572]
sample_names = []

for i in [0, 1, 2]:
    sample_name = "sample_" + str(sample_ids[i])

['sample_100', 'sample_231', 'sample_572']

The example code above differs from our first attempt in several ways.
Let's walk through what each of these changes is doing: - We created an empty sample_names list.
Rather than creating a new variable for each sample name we're creating, we can store those values in a list that mirrors our sample_ids list.
We create this empty list so that we have somewhere to put the new strings we're generating. We add those new values to the empty list using the .append() method.

  • We used a for loop to iterate through the original list.
    In the original example, we manually added the string 'sample_' and converted the int from sample_ids to a str. We accessed the values in sample_ids using the explicit index at each position, e.g. [0], [1], [2].
    In the revised code, we instead use the for loop to do that tedious work for us. The loop assigns a variable i to a list of values, [0, 1, 2]. Every time the loop runs, it replaces the variable i in our code with one of the values in the list, in that exact order.

Hopefully you can see from this example how loops can help us automate repetitive processes. This also helps our code become much more readable.

2.2 Implicit iteration

In a lot of programming languages, looping requires you to create a separate list of elements that are used for iteration, as in the previous example.

Python actually has a more helpful way of looping over elements - rather than creating a separate list, you can simply iterate over the list you care about. See the example below, where we want to print out the elements in the list vegetables:

vegetables = ["pumpkin", "potato", "tomato"]

for veg in vegetables:

In the case above, instead of an arbitrary list [0, 1, 2], we use the specific vegetables list we'd like to iterate across. Instead of the variable i, we create a different variable, veg, which takes on the value of each element of vegetables in each iteration of the loop.

When iterating in this way, it's usually helpful to have a more explicit name such as veg than an unintuitive name such as i, as it helps contextualize what you're doing.

2.3 Loops using dicts

In addition to iterating through loops using list data structures, we can also use dicts. You can understand some of the behavior of how this iteration happens using the example below:

favorite_vegetables = {"Marko": "pumpkin", "Charlene": "potato", "Stu": "tomato"}

for entry in favorite_vegetables:

You'll notice that iterating using a for loop across a dictionary iterates over the keys of that dictionary.
If you wanted to access both the key and the value in the dictionary, you can use the .items() method of a dict data structure.

for villager, veg in favorite_vegetables.items():
    print(villager + "'s favorite vegetable is", veg + ".")
Marko's favorite vegetable is pumpkin.
Charlene's favorite vegetable is potato.
Stu's favorite vegetable is tomato.

Rather than using a single variable such as entry, because .items() returns two values for every entry in the dictionary, we can unpack those paired values into two separate variables: villager, and veg.

2.4 while loops

Another type of helpful loop in Python is the while loop. Unlike the for loop, which requires you to explicitly state what you're iterating over, a while loop continues as long as a conditional statement is True. For example:

a = [0, 1, 2, 3, 4]
i = 0

while a[i] < 3:
    i += 1    # The += operator adds the value on the right and updates the original variable.

In the above example, there are a number of different things happening.
Prior to the loop, we specify a variable i. At the end of each entry in the loop, we increment i by 1 in order to step through the list a.

How do we know when to stop? Take a look at the while statement:

while a[i] < 3:
This means that as long as the entry at index [i] in list [a] is less than 3, we continue through the loop, printing the value of a[i].

As soon as we arrive at i = 3, at the top of the while loop, we get the value a[i] = 3, which is NOT less than 3. Therefore, the while loop terminates, and the value 3 is never printed.

while loops can be very powerful for a class of Python functions that rely on recursion, but you can also easily end up writing functions that "hang", or run forever without stopping.

P2. Practice

Let's explore using loops.

Practice 3

Write a function called fasta_renamer() which takes a list of filenames and renames them with the following specifications: - Converts all characters to lowercase. - Converts files ending with .fasta to .fa. - Optionally, adds a prefix of the user's choice to the start of every file name. - Returns a list of the new file names.

Note: In this case, we're performing this operation on a list of file name strings, not actual files.
In the next lesson, we'll talk about actually reading and writing files.

Practice 3 Sample Answer
def fasta_renamer(lst, prefix = ''):
    new_names = []
    for name in lst:
        new_name = name.lower().replace( '.fasta', '.fa')
        new_name = prefix + new_name
    return new_names
# Write your function in the space below. #
## Then run the cell to check your work. ##


starting_names = ['Chlamy.fa', 'ENSARG005.FASTA', 'Homo_sapiens.FA']
prefix = 'genome_'

fasta_renamer(starting_names, prefix)
['genome_chlamy.fa', 'genome_ensarg005.fa', 'genome_homo_sapiens.fa']

3. Booleans

In the previous lesson, we covered some basic data types such as int, float, and str. In this lesson, we've also covered data structures such as list and dict. One major data type that we haven't yet discussed is the bool, or Boolean.

bool values are True or False and are crucial to a lot of what gives programming languages power and flexibility to manage different types of scenarios.

bool values also have their own specific operands which can be used to build logic into our code. You can see some examples in the code below.

3.1 Equal

To check if two values are equivalent, you can use the == operand.
This evaluates to True if the two values are equal and False if they are not.

print(1 == 1)

print(1 == '1')

x = 'apple'
print(x == 'apple')

3.2 Not Equal

The opposite of the == operand is the != operand, which returns True if the values are NOT equal.

print(1 != 1)

print(1 != '1')

3.3. Greater Than or Less Than

You can also perform comparison between two values using expressions such as >, <, >=, and <=.

print(1 < 2)

print(1 > 2)

print(2 <= 2.0)

3.4 and and or

You can also chain together boolean expressions using the operands and and or. - The and operator only returns True if BOTH of the elements are True. - the or operator returns True if EITHER of the elements are True.

print(1 == 1 and 2 == 2)

print(1 == 1 or 2 == 3)

3.5 not

The not operand gives the inverse of a boolean expression.

print(not (1 == 1 and 2 == 2))

3.6 Checking membership with in

The in operand searches for values in data structures such as list or dict.
It can also be used to search for substrings within str.

pasture = ['sheep', 'sheep', 'sheep', 'wolf', 'sheep', 'sheep']

print('wolf' in pasture)

sentence = 'The quick brown fox jumped over the lazy dog.'

print('dog' in sentence)
print('monkey' in sentence)

These Boolean operations are crucial to the logic that underpins programming. They are also the backbone of conditional expressions, which allow us to have a Python script make decisions.

4. Conditional expressions

When working with data, you don't always want to perform the same transformations to every feature of the data. Sometimes, you want to select only specific aspects of the data and manipulate those. Using boolean expressions, we can generate conditional code.

For example, we could determine whether a value falls within a specific range using an if statement.

4.1. if this, else that

We can use if statements to choose how Python will proceed, as in the example below.

value = 10

if value <= 7:
    print('Value is less than or equal to 7.')
elif value > 7 and value < 12:
    print('Value is between 7 and 12.')
    print('Value is greater than 12.')
Value is between 7 and 12.

In the example above, you can see the three major types of conditional expressions associated with an if statement: - if: If this expression evaluates as True, do the thing in the first indented block. - elif: If the if statement is False, and this is True, do the thing in the second indented block. - else: If all other if and elif statements are false, do the thing in the last indented block.

4.2 Bringing it all together

The above example might seem a bit silly, because we can already tell in advance where 10 is relative to 7 and 12.

But you could also write this code in a different way to be able to set the values of the high and low bar.

value = 10

upper = 18
lower = 10

if value <= lower:
    print(value, 'is less than or equal to', str(lower) + '.')
elif value > lower and value < upper:
    print(value, 'is between', str(lower), 'and', str(upper) + '.')
    print(value, 'is greater than', str(upper) + '.')
10 is less than or equal to 10.

You could make this code more reusable by turning it into a function.

def goldilocks(upper, lower, value):
    if value <= lower:
        print(value, 'is less than or equal to', str(lower) + '.')
    elif value > lower and value < upper:
        print(value, 'is between', str(lower), 'and', str(upper) + '.')
        print(value, 'is greater than', str(upper) + '.')
    return None

value = 10
upper = 30
lower = 5

goldilocks(upper, lower, value)
10 is between 5 and 30.

And, if you combined this function call with a for loop, you could easily apply the same function to a variety of different numbers.

values = [5, 6, 10, 30, 60, 1]
upper = 20
lower = 7

for value in values:
    goldilocks(upper, lower, value)
5 is less than or equal to 7.
6 is less than or equal to 7.
10 is between 7 and 20.
30 is greater than 20.
60 is greater than 20.
1 is less than or equal to 7.

4.3 Doing nothing

Sometimes, when combining a conditional and a loop, you want the loop to do nothing at all.
In this specific situation, you can force a loop to continue.

For example, let's say you had a list of names and wanted to add a number to only the ones beginning with 'C' and return those.

names = ['Celine', 'Britney', 'Stefani', 'Rina', 'Charli', 'Billie']
output = []

for name in names:
    if name[0] == 'C':
        output.append(name + '_01')

['Celine_01', 'Charli_01']

4.4 Other conditional statements

There are other types of conditional statements in Python which you'll encounter less frequently, but can still be very helpful:

  • try except finally: tries to do something, unless it returns an error.
    If an error occurs, runs the except statement. You can specify different outcomes for different error types.
    finally at the end of each attempt, do a thing.
values = ['a', 1, 3, '5', 'q', 3.0]

for value in values:
        print(value / 3)
    except TypeError:
        print("Cannot divide - wrong value type.")
Cannot divide - wrong value type.
Cannot divide - wrong value type.
Cannot divide - wrong value type.
  • match case: works very similarly to an if - elif - else statement, but can be much easier to read when you have specific expected outcomes. See example here.

NOTE: match - case statements are still very new to Python 3.10, so Jupyter doesn't have syntax highlighting for them yet.

  • with open(): used for opening files. We'll cover this in lesson 3!

P3. Practice

Let's consider how we might use conditional expressions to help us navigate biological data.
A common case would be grouping data starting with similar IDs into a dictionary structure.

Practice 4

You have a list of filenames that you want to aggregate into a dictionary based on the IDs contained within them.
Write a function called aggregate_files() with the following behavior: - The function accepts a list and groups elements of the list into a dictionary based on a prefix. - You can assume that each file starts with a prefix delimited by a _. - The function should return a dictionary. - Each entry in the dictionary should be a list containing elements from the input list. - Hint: You could try using conditional expressions or the set datatype.

Practice 4 Sample Answer
def aggregate_files(file_list):
    output_dict = {}
    for file in file_list:
        prefix = file.split('_')[0]
        if prefix not in  output_dict.keys():
            output_dict[prefix] = [file]
            output_dict[prefix] = output_dict[prefix] + [file]
    return output_dict
# Write your function in the space below. #
## Then run the cell to check your work. ##


file_names = ['Chlamy_img001.tiff', 'Chlamy_img002.tiff', 'Chlamy_img003.tiff', 'Chlamy_img004.tiff', 
              'Chlamy_img005.tiff', 'Colpoda_img001.tiff', 'Colpoda_img002.tiff', 'Colpoda_img003.tiff', 
              'Bigelow_img001.tiff', 'Bigelow_img002.tiff', 'Bigelow_img003.tiff', 'LEX_img001.tiff']

{'Chlamy': ['Chlamy_img001.tiff',
 'Colpoda': ['Colpoda_img001.tiff',
 'Bigelow': ['Bigelow_img001.tiff',
 'LEX': ['LEX_img001.tiff']}

5. Problem Set

With a wealth of data types, functions, loops, and conditional expressions at your disposal, you can accomplish many different computations using Python.

This week's problem set features several open-ended problems that you can solve in many different ways.
Some will revisit and revise the code you wrote in Problem Set 1.
Hopefully you'll be able to write some fun and interesting code and feel empowered by Python programming!

You can find the problem set at this Google CoLab Document.
CoLab works just like a Jupyter notebook, but it automatically saves your changes and keeps them as a cloud document.
To get started on the problem set, make sure to first click "File" > "Save a copy in Drive" to make sure you don't lose your progress!

We'll have AUG office hours next week for you to stop by and work on your problem set with other Arcadians as well as to work through any bugs in your code.
If you're stumped, we also have an Answer Key that you can check.

Feel free to ping the #software-questions Slack channel for anything that comes up in the meantime!