Send Single SMS with Python and Flask

Overview

Build a production-ready Flask endpoint that sends SMS messages using the Telnyx Python SDK. This tutorial demonstrates the new client-based initialization pattern, proper error handling for telecom APIs, and secure credential management via environment variables.

Prerequisites

• Python 3.8 or higher.
• A Telnyx account with an active API key from the Telnyx Portal.
• A Telnyx phone number enabled for outbound SMS.
• pip (Python package manager).

Step 1: Setup

Install the required dependencies:

pip install telnyx flask python-dotenv

Create a project directory and navigate into it:

mkdir telnyx-sms-sender
cd telnyx-sms-sender

Step 2: Configuration

Create a .env file in your project root to store credentials securely:

TELNYX_API_KEY=YOUR_API_KEY_HERE
TELNYX_PHONE_NUMBER=+15551234567

Replace YOUR_API_KEY_HERE with your actual API key and +15551234567 with your Telnyx phone number in E.164 format.

Step 3: Implementation

Create app.py and initialize the Telnyx client using the new pattern. Define a helper function to handle message creation with proper validation:

import os
import telnyx
from dotenv import load_dotenv

load_dotenv()

# Initialize client with the new SDK pattern
client = telnyx.Telnyx(api_key=os.getenv("TELNYX_API_KEY"))


def send_sms(to_number: str, message: str) -> dict:
    """Send SMS via Telnyx and return JSON-serializable response data."""
    from_number = os.getenv("TELNYX_PHONE_NUMBER")
    if not from_number:
        raise ValueError("TELNYX_PHONE_NUMBER environment variable not set")

    # Validate E.164 format to prevent API errors
    if not to_number.startswith("+"):
        raise ValueError("Phone number must be in E.164 format (e.g., +15551234567)")

    # Use client.messages.create() with proper parameters
    response = client.messages.create(
        from_=from_number,
        to=to_number,
        text=message,
    )

    # Extract serializable data — SDK objects are NOT JSON-serializable
    return {
        "message_id": response.data.id,
        "status": response.data.to[0].status if response.data.to else "unknown",
        "from": from_number,
        "to": to_number,
    }

Step 4: Testing

Add the Flask route with comprehensive error handling for production resilience:

from flask import Flask, jsonify, request

app = Flask(__name__)


@app.route("/sms/send", methods=["POST"])
def send_sms_endpoint():
    """HTTP endpoint to send single SMS."""
    data = request.get_json()

    if not data:
        return jsonify({"error": "Request body required"}), 400

    to_number = data.get("to")
    message = data.get("message")

    if not to_number or not message:
        return jsonify({"error": "Missing required fields: 'to' and 'message'"}), 400

    try:
        result = send_sms(to_number, message)
        return jsonify(result), 200

    except telnyx.AuthenticationError:
        return jsonify({"error": "Invalid API key"}), 401
    except telnyx.RateLimitError:
        return jsonify({"error": "Rate limit exceeded. Please slow down."}), 429
    except telnyx.APIStatusError as e:
        return jsonify({"error": str(e), "status_code": e.status_code}), e.status_code
    except telnyx.APIConnectionError:
        return jsonify({"error": "Network error connecting to Telnyx"}), 503
    except ValueError as e:
        return jsonify({"error": str(e)}), 400


if __name__ == "__main__":
    app.run(debug=True, port=5000)

Start the server:

python app.py

Test the endpoint using curl:

curl -X POST http://localhost:5000/sms/send \
  -H "Content-Type: application/json" \
  -d '{"to": "+15559876543", "message": "Hello from Telnyx!"}'

Expected response:

{
  "message_id": "403171d8-8b00-4b8f-9e9c-1234567890ab",
  "status": "queued",
  "from": "+15551234567",
  "to": "+15559876543"
}

Complete Code

#!/usr/bin/env python3
"""Production-ready Flask endpoint for sending SMS via Telnyx."""

import os
import telnyx
from dotenv import load_dotenv
from flask import Flask, jsonify, request

load_dotenv()

app = Flask(__name__)

# Initialize client with the new SDK pattern
client = telnyx.Telnyx(api_key=os.getenv("TELNYX_API_KEY"))


def send_sms(to_number: str, message: str) -> dict:
    """Send SMS via Telnyx and return JSON-serializable response data."""
    from_number = os.getenv("TELNYX_PHONE_NUMBER")
    if not from_number:
        raise ValueError("TELNYX_PHONE_NUMBER environment variable not set")

    # Validate E.164 format to prevent API errors
    if not to_number.startswith("+"):
        raise ValueError("Phone number must be in E.164 format (e.g., +15551234567)")

    # Use client.messages.create() with proper parameters
    response = client.messages.create(
        from_=from_number,
        to=to_number,
        text=message,
    )

    # Extract serializable data — SDK objects are NOT JSON-serializable
    return {
        "message_id": response.data.id,
        "status": response.data.to[0].status if response.data.to else "unknown",
        "from": from_number,
        "to": to_number,
    }


@app.route("/sms/send", methods=["POST"])
def send_sms_endpoint():
    """HTTP endpoint to send single SMS."""
    data = request.get_json()

    if not data:
        return jsonify({"error": "Request body required"}), 400

    to_number = data.get("to")
    message = data.get("message")

    if not to_number or not message:
        return jsonify({"error": "Missing required fields: 'to' and 'message'"}), 400

    try:
        result = send_sms(to_number, message)
        return jsonify(result), 200

    except telnyx.AuthenticationError:
        return jsonify({"error": "Invalid API key"}), 401
    except telnyx.RateLimitError:
        return jsonify({"error": "Rate limit exceeded. Please slow down."}), 429
    except telnyx.APIStatusError as e:
        return jsonify({"error": str(e), "status_code": e.status_code}), e.status_code
    except telnyx.APIConnectionError:
        return jsonify({"error": "Network error connecting to Telnyx"}), 503
    except ValueError as e:
        return jsonify({"error": str(e)}), 400


if __name__ == "__main__":
    app.run(debug=True, port=5000)

Troubleshooting

Next Steps

• Receive SMS Webhooks with Python.
• Send Bulk SMS Messages.
• Build Two-Factor Authentication with SMS.