Conditional Logic in Methods

Real data has gaps. A method needs to behave sensibly when the collection is empty, a value is missing, or a search finds nothing at all.

1. Learning Objective

2. Pre-Coding Quiz

Answer these before reading the concept explanation or writing any code. It is fine to get these wrong — that is the point. You need 4/5 to proceed to the lab.

3. The Concept — Conditional Logic in Methods

Branching based on instance state

A method can look at self's current attributes and behave differently depending on what it finds — exactly like the plain if-statements from earlier sessions, just now reading self instead of a passed-in argument.

class CountryExplorer:
    def __init__(self, countries):
        self.countries = countries

    def status_message(self):
        if not self.countries:
            return "No countries loaded yet."
        return f"{len(self.countries)} countries loaded."

empty_explorer = CountryExplorer(countries=[])
print(empty_explorer.status_message())  # "No countries loaded yet."

Truthy and falsy collections

Python treats an empty list, empty string, empty dict, 0, and None as falsy in a boolean context. A non-empty version of any of them is truthy. This lets you write if not self.countries: instead of the more verbose if len(self.countries) == 0:.

countries = []
print(bool(countries))  # False — empty list is falsy

countries = [1]
print(bool(countries))  # True — non-empty list is truthy

# Idiomatic style
if not countries:
    print("empty")
else:
    print("has items")

The ternary expression for short either/or values

When a value simply needs to be one of two things based on a condition, a ternary expression is more compact than a full if/else block, and it produces a value directly rather than a side effect.

def region_or_unknown(country):
    return country.region if country.region else "Unknown"

# equivalent longhand
def region_or_unknown_longhand(country):
    if country.region:
        return country.region
    else:
        return "Unknown"

"Not found" is not the same as "broken"

Recall Session 11: exceptions are for genuinely invalid states. A search that legitimately finds nothing should return an empty, valid result — not raise an exception. Confusing the two forces every caller to wrap normal searches in try/except unnecessarily.

class CountryExplorer:
    def __init__(self, countries):
        self.countries = countries

    def find_by_region(self, region):
        # Zero matches is a NORMAL outcome — return an empty list, don't raise
        return [c for c in self.countries if c.region == region]

explorer = CountryExplorer(countries=[])
result = explorer.find_by_region("Antarctica")
print(result)  # [] — a valid, empty answer, no crash, no exception

4. Lab

What you will build

A file called conditional_lab.py, building on Session 16's CountryExplorer.

Step-by-step instructions

# conditional_lab.py
class Country:
    def __init__(self, name, region, population):
        self.name = name
        self.region = region
        self.population = population

    def summary(self):
        return f"{self.name} ({self.region}): pop. {self.population:,}"


class CountryExplorer:
    def __init__(self, countries):
        self.countries = countries

class CountryExplorer:
    def __init__(self, countries):
        self.countries = countries

    def status_message(self):
        if not self.countries:
            return "No countries loaded yet."
        return f"{len(self.countries)} countries loaded."

empty = CountryExplorer(countries=[])
loaded = CountryExplorer(countries=[Country(name="Kenya", region="Africa", population=54000000)])
print(empty.status_message())
print(loaded.status_message())

class CountryExplorer:
    def __init__(self, countries):
        self.countries = countries

    def status_message(self):
        if not self.countries:
            return "No countries loaded yet."
        return f"{len(self.countries)} countries loaded."

    def find_by_region(self, region):
        return [c for c in self.countries if c.region == region]

explorer = CountryExplorer(countries=[Country(name="Kenya", region="Africa", population=54000000)])
result = explorer.find_by_region("Antarctica")
print(result)          # []
print(type(result))    # <class 'list'> — never None, never an exception

class Country:
    def __init__(self, name, region, population, capital=None):
        self.name = name
        self.region = region
        self.population = population
        self.capital = capital

    def display_capital(self):
        return self.capital if self.capital else "Unknown"

k = Country(name="Kenya", region="Africa", population=54000000, capital="Nairobi")
u = Country(name="Unclaimed", region="Antarctica", population=0)
print(k.display_capital())  # "Nairobi"
print(u.display_capital())  # "Unknown"

explorer = CountryExplorer(countries=[])
print(explorer.status_message())  # "No countries loaded yet."
explorer.countries.append(k)
print(explorer.status_message())  # "1 countries loaded." — note the grammar, discuss in reflection

5. Expected Files Changed

6. Commit Checkpoint

Once the lab is complete and you can explain every line, make this exact commit:

git add conditional_lab.py docs/sessions/session-17/index.html
git commit -m "session-17: handle empty collections and missing values with conditional logic"

7. Code Review Checklist

Go through your code line by line and check each item:

8. Post-Coding Quiz

Same 5 questions. Take it again now that you have written and run the code. You need 4/5 to mark this session complete.

9. Reflection Questions

Think through these after the post-quiz. No right answer — they are for discussion.

10. What Breaks If This Knowledge Is Missing?

11. What We Learned