Python for Network Engineers

Python is the dominant programming language for network automation. Its clear syntax, rich standard library, and thriving ecosystem of networking libraries make it the natural choice for engineers who need to automate repetitive tasks.

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Why Python?

Reason	Detail
Readable syntax	Looks like pseudocode, easy to learn for non-programmers
Rich ecosystem	Netmiko, NAPALM, Paramiko, pyATS, Nornir, and more
Cross-platform	Runs on Linux, macOS, and Windows
Community	Massive community with tutorials, forums, and open-source projects
Vendor support	Cisco, Arista, Juniper, and others provide Python SDKs and libraries
Rapid prototyping	Write a working script in minutes, not hours

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Python Fundamentals for Networking

Variables and Data Types

# Strings — device hostnames, IP addresses, commands
hostname = "switch01"
ip_address = "192.168.1.1"

# Integers — port numbers, VLAN IDs
vlan_id = 100
ssh_port = 22

# Booleans — feature flags
is_enabled = True

# Lists — collections of items (ordered, mutable)
interfaces = ["GigabitEthernet0/1", "GigabitEthernet0/2", "GigabitEthernet0/3"]

# Dictionaries — key-value pairs (like a device inventory)
device = {
    "hostname": "switch01",
    "ip": "192.168.1.1",
    "platform": "cisco_ios",
    "username": "admin",
    "password": "secret123",
}

Loops

# Loop through a list of devices
devices = ["192.168.1.1", "192.168.1.2", "192.168.1.3"]

for ip in devices:
    print(f"Connecting to {ip}...")

# Loop through a dictionary
device = {"hostname": "R1", "ip": "10.0.0.1", "os": "ios"}

for key, value in device.items():
    print(f"{key}: {value}")

Conditionals

# Check device type and choose the right driver
platform = "cisco_ios"

if platform == "cisco_ios":
    print("Using Netmiko cisco_ios driver")
elif platform == "arista_eos":
    print("Using Netmiko arista_eos driver")
else:
    print(f"Unsupported platform: {platform}")

Functions

def get_device_config(ip: str, username: str, password: str) -> str:
    """Connect to a device and retrieve its running configuration."""
    # Connection logic would go here
    print(f"Retrieving config from {ip}")
    return "! Running configuration..."

# Call the function
config = get_device_config("192.168.1.1", "admin", "secret")

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Working with Files

Network automation frequently involves reading from and writing to files:

# Read a list of device IPs from a file
with open("devices.txt", "r") as f:
    devices = [line.strip() for line in f if line.strip()]

print(f"Loaded {len(devices)} devices")

# Write a device configuration to a file
config = "hostname switch01\ninterface Gi0/1\n no shutdown"

with open("backups/switch01.cfg", "w") as f:
    f.write(config)

print("Configuration saved")

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Working with JSON and YAML

JSON

import json

# Parse JSON from a string
json_string = '{"hostname": "R1", "interfaces": ["Gi0/0", "Gi0/1"]}'
data = json.loads(json_string)
print(data["hostname"])  # R1

# Read JSON from a file
with open("inventory.json", "r") as f:
    inventory = json.load(f)

# Write JSON to a file
with open("output.json", "w") as f:
    json.dump(data, f, indent=2)

YAML

import yaml

# Read YAML from a file
with open("inventory.yaml", "r") as f:
    inventory = yaml.safe_load(f)

# Write YAML to a file
with open("output.yaml", "w") as f:
    yaml.dump(inventory, f, default_flow_style=False)

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Error Handling

Network operations are inherently unreliable — devices may be unreachable, credentials may fail, and timeouts happen: