Store and Return the Expense

Assign an identifier, store the expense, and return it to the client

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The global keyword

You defined counter = 0 at the top of your file, outside any function. This makes it a module-level variable — it exists for the entire program, not just inside one function.

Python functions can read module-level variables without any special syntax. But when you try to modify one with counter += 1, Python assumes you mean a local variable that was never defined and raises an UnboundLocalError.

The global keyword fixes this. It tells Python: "don't create a new local variable — use the existing module-level one."

def create_expense(expense: Expense):
    global counter
    counter += 1  # modifies the module-level counter

You only need global when you reassign the variable. Reading expenses[counter] works fine without it because you are modifying the dictionary's contents, not replacing the variable itself.

Convert a Pydantic model to a dictionary

The model_dump() method converts a Pydantic model into a plain Python dictionary:

expense.model_dump()
# {"description": "Lunch", "amount": 12.5, "category": "food", "date": None}

This gives you a regular dictionary you can modify freely. The Pydantic model's job is done after validation.

Add the identifier with the spread operator

The ** spread operator unpacks a dictionary into key-value pairs. You can use it to create a new dictionary that combines existing fields with new ones:

{"id": counter, **expense.model_dump()}
# {"id": 1, "description": "Lunch", "amount": 12.5, "category": "food", "date": None}

This creates a single dictionary with the id field first, followed by all the expense fields. Storing this in expenses[counter] lets you look up any expense by its identifier later.

Instructions

Fill in the create_expense function body to assign an identifier, store the expense, and return it.

Add global counter to access the module-level counter.
Increment counter by 1.
Store the expense in the dictionary with expenses[counter] = {"id": counter, **expense.model_dump()}.
Return expenses[counter].

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from datetime import datetime
from fastapi import FastAPI
from pydantic import BaseModel, Field, field_validator
from typing import Literal, Optional

app = FastAPI()

class Expense(BaseModel):
    description: str = Field(min_length=1)
    amount: float = Field(gt=0)
    category: Literal["food", "transport", "entertainment", "utilities", "other"]
    date: Optional[str] = None

    @field_validator("date")
    @classmethod
    def validate_date_format(cls, v):
        if v is not None:
            try:
                datetime.strptime(v, "%Y-%m-%d")
            except ValueError:
                raise ValueError("Date must be in YYYY-MM-DD format")
        return v

expenses = {}
counter = 0

@app.post("/expenses", status_code=201)
def create_expense(expense: Expense):
    # Step 1: Add global counter
    # Step 2: Increment counter by 1
    # Step 3: Store expenses[counter] = {"id": counter, **expense.model_dump()}
    # Step 4: Return expenses[counter]

Store and Return the Expense

The global keyword

Convert a Pydantic model to a dictionary

Add the identifier with the spread operator

Instructions

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