Unit 7 — Input and Output — Reading and Writing Network Data

Reading Text Files

Most real network data starts as a file — a device list, a config export, a log. Python reads files with open():

with open("devices.txt") as f:
    for line in f:
        print(line.strip())

with ensures the file closes automatically when the block ends. strip() removes the newline at the end of each line.

To read the whole file at once:

with open("devices.txt") as f:
    content = f.read()

To read into a list of lines:

with open("devices.txt") as f:
    lines = f.readlines()

Writing Files

Open with "w" to write (creates the file if it does not exist, overwrites if it does). Open with "a" to append:

with open("report.txt", "w") as f:
    f.write("Device Status Report\n")
    f.write("=" * 25 + "\n")
    f.write("sw-core-01 — online\n")

Combine reading and writing to transform data:

with open("devices.txt") as infile, open("report.txt", "w") as outfile:
    for i, line in enumerate(infile, start=1):
        outfile.write(f"{i:2}. {line.strip()}\n")

Formatted Output with f-strings

You have used f-strings already. A few more tools:

Alignment and padding:

name   = "sw-core-01"
status = "online"
print(f"{name:<20} {status}")
# sw-core-01           online

:<20 left-aligns and pads to 20 characters. :>20 right-aligns. :^20 centres.

Number formatting:

accuracy = 0.9375
print(f"Accuracy: {accuracy:.1%}")
# Accuracy: 93.8%

count = 1234567
print(f"Packets: {count:,}")
# Packets: 1,234,567

JSON — The Format Every Network API Uses

JSON is what network APIs return. Python's json module reads and writes it.

Save a list of device dicts to a file:

import json

devices = [
    {"name": "sw-core-01", "ip": "10.1.1.1", "site": "HQ"},
    {"name": "rtr-edge-01", "ip": "10.1.1.254", "site": "HQ"},
]

with open("devices.json", "w") as f:
    json.dump(devices, f, indent=2)

Load it back:

with open("devices.json") as f:
    devices = json.load(f)

for d in devices:
    print(d["name"], "—", d["ip"])

json.dump() writes to a file. json.dumps() returns a string. json.load() reads from a file. json.loads() parses a string.

Summary

• open("file") reads, open("file", "w") writes, open("file", "a") appends
• Always use with — it closes the file automatically
• f.read() = whole file as string, f.readlines() = list of lines, for line in f = one line at a time
• strip() removes trailing newlines when reading line by line
• json.dump() writes JSON to a file, json.load() reads it back
• f-string padding: {value:<20} left-align, {value:>20} right-align