API error handling in Flask - Step-by-Step Execution

Concept Flow - API error handling

Client sends request

↓

API receives request

↓

Process request

↓

Error occurs?

No→Return success response

Yes↓

Catch error

↓

Return error response with status code

↓

Client receives error info

The API receives a request, processes it, and if an error happens, it catches it and sends back an error response with a status code.

Execution Sample

Flask

from flask import Flask, jsonify
app = Flask(__name__)

@app.route('/divide/<int:a>/<int:b>')
def divide(a, b):
    try:
        result = a / b
        return jsonify({'result': result})
    except ZeroDivisionError:
        return jsonify({'error': 'Cannot divide by zero'}), 400

This API endpoint divides two numbers and returns the result or an error if dividing by zero.

Execution Table

Step	Input (a,b)	Action	Result	Response Sent
1	a=10, b=2	Try division	10 / 2 = 5.0	{'result': 5.0}, status 200
2	a=5, b=0	Try division	ZeroDivisionError raised	{'error': 'Cannot divide by zero'}, status 400
3	a=7, b=1	Try division	7 / 1 = 7.0	{'result': 7.0}, status 200
4	a=3, b=0	Try division	ZeroDivisionError raised	{'error': 'Cannot divide by zero'}, status 400
5	-	No more requests	-	-

💡 Execution stops after all requests are processed.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
a	-	10	5	7	3	-
b	-	2	0	1	0	-
result	-	5.0	Error	7.0	Error	-
response	-	{'result': 5.0}, 200	{'error': 'Cannot divide by zero'}, 400	{'result': 7.0}, 200	{'error': 'Cannot divide by zero'}, 400	-

Key Moments - 2 Insights

Why does the API return a 400 status code when dividing by zero?

Because the ZeroDivisionError is caught and the API sends a JSON error message with status 400, as shown in execution_table rows 2 and 4.

What happens if no error occurs during division?

The API returns the division result in JSON with status 200, as seen in execution_table rows 1 and 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the response when a=5 and b=0?

A {'result': 1.0}, status 200

B {'result': 5.0}, status 200

C {'error': 'Cannot divide by zero'}, status 400

D No response

💡 Hint

Check row 2 in the execution_table where b=0 causes an error response.

At which step does the division succeed without error?

A Step 2

B Step 1

C Step 4

D Step 5

💡 Hint

Look for steps where 'result' is a number, not 'Error' in variable_tracker.

If we remove the try-except block, what would happen when b=0?

A API crashes and returns a server error (500)

B API returns a JSON error with status 400

C API returns result 0

D API returns status 200 with empty response

💡 Hint

Without catching ZeroDivisionError, Python raises an exception causing a server error.

Concept Snapshot

API error handling in Flask:
- Use try-except inside route functions
- Catch specific errors (e.g., ZeroDivisionError)
- Return JSON error message with HTTP status code
- Normal responses return data with status 200
- Helps client understand what went wrong

Full Transcript

This example shows how a Flask API handles errors by using try-except blocks inside route functions. When dividing two numbers, if the divisor is zero, a ZeroDivisionError occurs. The API catches this error and returns a JSON response with an error message and status code 400. If no error occurs, it returns the division result with status 200. This approach helps clients know if their request failed and why, improving communication and reliability.