APIs Explained — A Practical Guide for Finance & Business Leaders

2025-09-02 • Python

1. Why APIs Matter in Finance

Every time your trading app fetches live stock prices, your accounting tool syncs invoices, or your dashboard updates FX rates — there’s an API working behind the scenes.

Think of an API as a secure messenger:

You (the client) send a request: “Give me Tesla’s latest stock price.”
The server receives it, looks it up, and replies with a structured response: “Here is the price: 250.10 USD.”

APIs let different systems talk without you manually exporting/importing files.

2. What Exactly Is an API?

API = Application Programming Interface.

Application: The system providing data or services (Bloomberg, Yahoo Finance, your ERP).
Programming: The structured way we ask questions (HTTP, JSON).
Interface: The “middleman” between your software and the service.

3. Options & Types of APIs

There are several API styles. The most relevant for finance & business:

API Style	How It Works	Pros	Cons	Common Use in Finance
REST	Request/response over HTTP. Structured JSON.	Simple, widely supported	Sometimes “over-fetches” data	Market data, payment gateways, ERP connectors
GraphQL	You specify exactly what fields you want.	Flexible, efficient	More complex to learn	Some fintech dashboards, flexible analytics
SOAP	XML-based protocol (older).	Secure, robust	Heavy, verbose	Legacy banking & insurance systems
Webhooks	The server pushes data to you.	Real-time updates	Less control	Notifications (payments, trades)

4. REST in Action (Simple Example)

REST APIs are the most common. Let’s fetch the latest Bitcoin price from a free service:

import requests

url = "https://api.coindesk.com/v1/bpi/currentprice/BTC.json"
res = requests.get(url)

data = res.json()
print("Bitcoin Price:", data["bpi"]["USD"]["rate"])

✅ Key points:

requests.get(url) → send a request
res.json() → parse the response into structured Python
You get a usable number (62,450.23) instead of a web page.

5. FastAPI Example (Building Your Own API)

APIs aren’t just for big players — you can build your own for internal finance teams.
Let’s make a simple API that returns exchange rates.

# save as exchange_api.py
from fastapi import FastAPI

app = FastAPI()

@app.get("/fx/{pair}")
def get_rate(pair: str):
    rates = {"USD-EUR": 0.91, "USD-AED": 3.67}
    return {"pair": pair, "rate": rates.get(pair, "Not available")}

Run it locally:

uvicorn exchange_api:app --reload

Now you can open:

http://127.0.0.1:8000/fx/USD-EUR

It returns:

{"pair": "USD-EUR", "rate": 0.91}

✅ Use cases:

Build an internal API to serve finance dashboards
Wrap Excel/ERP data so analysts can fetch it programmatically
Standardize reference data across departments

6. REST vs FastAPI: Key Differences

Aspect	REST (concept)	FastAPI (framework)
Definition	A style of APIs using HTTP + JSON	A Python framework for building APIs
Learning curve	Very easy to consume	Easy to build, modern Python
Flexibility	Widely supported across all systems	Ideal for custom internal APIs
Finance context	Pulling data from external providers (e.g. Bloomberg, Yahoo, Coindesk)	Building custom APIs (e.g. exposing ERP data, internal FX rates)

🔑 Rule of thumb for finance teams:

Use REST to consume external APIs (market data, payments, reporting).
Use FastAPI when you want to create your own APIs for internal workflows.

7. Professional Use Cases for Finance Teams

Automation: Pull FX rates every morning into your treasury spreadsheet automatically.
Dashboards: Feed APIs into BI tools like Superset, Power BI, Tableau.
Integrations: Connect ERP (Oracle, SAP) to external data (market prices, tax rates).
Client Services: Expose APIs for your customers (e.g. balance inquiries, invoice status).

8. Key Takeaways

APIs = Secure, automated pipelines between systems.
REST is the most common and easiest to learn.
FastAPI is the modern choice for creating APIs in Python.
For finance, APIs mean less manual effort, faster reporting, and more accurate data.

✍️ Next steps:

Try calling a simple REST API in Python (like the BTC example).
If you’re ready, build your first FastAPI endpoint for something internal (e.g., your daily FX table).
Think: what in my workflow is repetitive that an API could automate?