Python Fundamentals — Lists, Tuples, Dictionaries, and Sets (Part I)

Before you begin, take a few minutes to check what you already know and how confident you feel. You will see the same questions again at the end of the module — this helps both you and us measure what you have learned. Click an option for every question and confidence rating, then click Next to continue.

Once you can store a single value in a variable, the next question is how to store many values together. This module covers the four built-in containers Python gives you for collecting data:

• Lists — ordered, changeable collections, like a column of patient ages.
• Tuples — ordered collections that cannot change after they are made, like a fixed (row, column) coordinate.
• Dictionaries — labelled collections that look things up by name, like a patient record where you ask for the age or the diagnosis directly.
• Sets — unordered collections of unique values, useful for finding what is in one group but not another.

Try every snippet in the Python Scratchpad on the right. By the end of the module you will pick the right container for the job, read a line of code that uses any of the four, and know the handful of methods you use to change them in Part II.

When you have a handful of values that belong together — the four ages of the patients in your cohort, the ten SNPs your colleague flagged, the rows of a small table — you reach for a list.

A list is a sequence of values inside square brackets, separated by commas.

For example, ages = [54, 67, 71, 49] stores four whole numbers under the name ages, and genes = ["APOE", "TREM2", "BIN1"] stores three text values under the name genes.

Here are some examples of how to build a list in Python.

ages = [54, 67, 71, 49]
genes = ["APOE", "TREM2", "BIN1"]
print(ages)
print(genes)

Lists keep the order you put things in. The first item is at position 0 (not 1), the second is at position 1, and so on. Negative positions count from the end: -1 is the last item, -2 is the second-to-last. This is called indexing.

Here is an example of you can do indexing with a list in Python. Try it out.

meds = ["aspirin", "insulin", "warfarin"]
print(meds[0])
print(meds[-1])
print(meds[-2])

Here’s another for practice.

temps_celsius = [37.2, 38.1, 38.9, 38.4, 37.6]
print(temps_celsius[0])
print(temps_celsius[2])
print(temps_celsius[-1])
print(temps_celsius[-2])

Lists can hold any mix of types — numbers, text, even other lists — but in practice you almost always keep one list to one type. A list of patient ages should be all ints; a list of gene names should be all strings. Mixing types is allowed, just rarely a good idea.

When you want a piece of a list rather than a single item, use a slice. The syntax is two positions separated by a colon — the start position (included) and the stop position (excluded). Leaving a side blank means "go all the way to that end".

Can you predict what the code below should produce?

snps = ["rs1", "rs2", "rs3", "rs4", "rs5"]
print(snps[1:3])
print(snps[:2])
print(snps[-2:])

Your turn. Try practicing slicing a Python list below.

antibiotics = ["amoxicillin", "doxycycline", "ciprofloxacin", "ceftriaxone", "vancomycin", "meropenem"]
print(antibiotics[:2])
print(antibiotics[-2:])
print(antibiotics[2:5])
print(antibiotics[::2])

Looping over a list is the bread and butter of data work.

Looping over a list means going through each item in the list one by one and doing something with it.

Here is how you can set up a loop on a list.

sbp_readings = [118, 142, 130, 155, 122]
for sbp in sbp_readings:
    if sbp >= 140:
        print(f"{sbp} mmHg: hypertensive")
    else:
        print(f"{sbp} mmHg: normal")

First, let’s try and familiarize yourself with the syntax first.

Try the code below to practice this skill.

hba1c_values = [5.4, 6.1, 7.8, 5.9, 8.5]
for hba1c in hba1c_values:
    if hba1c >= 6.5:
        print(f"{hba1c}%: diabetic range")
    else:
        print(f"{hba1c}%: not diabetic")

Tuples look almost the same as lists but use round brackets, and once you have made one you cannot change what is inside. You index and slice them the same way you do a list.

The reason to reach for a tuple instead of a list is when the collection has a fixed shape — a (row, column) pair, a (gene, p_value, significant) record — and you do not want anyone to accidentally change the parts later.

You can set up a tuple as below. Try it out.

patient = ("P-1042", 58, "O+", 1.72, 68.5)
patient_id, age, blood_type, height_m, weight_kg = patient
bmi = weight_kg / (height_m ** 2)
print(f"{patient_id}: BMI = {round(bmi, 1)}")

Like lists, you can slice tuples to extract data. Let’s practise slicing tuples!

hr_week = (72, 78, 75, 82, 70, 68, 74)
print(hr_week[0])
print(hr_week[-1])
print(hr_week[:3])
print(hr_week[-3:])
print(hr_week[::-1])

Can you predict the results of the code below?

snps = ("rs429358", "rs7412", "rs1801133", "rs6025", "rs1799963")

In summary, a list lets you change its contents (mutable) after creation, while a tuple does not (immutable). If you try to modify a tuple, Python throws a TypeError.

A dictionary stores pairs of keys and values inside curly braces. Keys are usually short strings; values can be anything — a number, a string, a list, even another dictionary.

For example, patient = {"id": "P001", "age": 54, "gene": "APOE"} stores three key-value pairs. You read a value back by writing the dictionary name and the key in square brackets: patient["age"] gives 54.

patient = {"id": "P001", "age": 54, "gene": "APOE"}
print(patient["id"])
print(patient["age"])
print(len(patient))

If you ask for a key that is not in the dictionary, Python raises a KeyError. There are two clean ways to avoid that.

• The first is to test for the key with the in operator: if "middle_name" in patient.
• The second is to use the .get() method, which returns None (or a default value of your choosing) instead of crashing.

patient = {"id": "P001", "age": 54}
print(patient["middle_name"])

Looping over a dictionary walks the keys by default.

• To loop over the key and the value at the same time, use .items().
• To loop over the values only, use .values().

Try the code below to see what we mean.

vitals = {"heart_rate": 72, "systolic_bp": 118, "temp_c": 36.8}
for measurement, value in vitals.items():
    print(f"{measurement}: {value}")
for v in vitals.values():
    print(v)

A set stores unique values with no duplicates and no order. You make one with curly braces — genes_a = {"APOE", "TREM2", "BIN1"} — but unlike a dictionary, you write only values, no keys. If you put a duplicate in, the set silently keeps only one copy.

Sets support three operators that read like the maths they implement.

• & is intersection (in both).
• | is union (in either).
• - is difference (in the first but not the second).

Let’s practise using the operators above on a set!

diabetes = {"P001", "P002", "P003", "P004"}
hypertension = {"P002", "P004", "P005", "P006"}
print(diabetes & hypertension)
print(diabetes | hypertension)
print(diabetes - hypertension)

You have reached the end of the module. Try the same questions again — your answers here, paired with your pre-test answers, are how we measure what the module taught you. Answer every question and confidence rating, then click Submit and see results to view your score.

Here is how your post-test answers compare with your pre-test answers. The pre/post pairing is the most reliable way to see what this module actually taught you.