Dictionaries (`dict`) + Looping Patterns for DevOps

1. Why `dict` is important

A dict stores data as key → value.

You use dictionaries in DevOps all the time:

Tags/labels: {"env": "prod", "owner": "devops"}
Metrics: {"service_auth": 129, "service_payment": 122}
JSON/YAML structures (APIs, Kubernetes manifests)
Inventory-like data: instances, servers, config objects

2. What is a dictionary?

A dictionary is a mapping:

Keys must be unique.
Keys are typically str (can also be int, tuple, etc.).
Values can be any type.

Example:

grades = {"guy": 50, "dylan": 80, "eitan": 90, "rotem": 100}

3. Creating dictionaries

3.1 Literal

d = {"env": "e2e", "owner": "all"}

3.2 Empty dict then fill

d = {}
d["env"] = "e2e"
d["owner"] = "all"

3.3 `dict()` constructor

d = dict(env="e2e", owner="all")

4. Reading values (access)

4.1 Using brackets `[]`

grades = {"guy": 50, "dylan": 80}
print(grades["guy"])

If the key doesn’t exist, this raises a KeyError:

print(grades["hen"])

4.2 Safe access with `.get()`

grades = {"guy": 50, "dylan": 80}
print(grades.get("hen"))
print(grades.get("hen", 0))

Use .get() when missing keys are normal.

5. Adding and updating values

5.1 Add a new key

grades = {"guy": 50}
grades["hen"] = 99
print(grades)

5.2 Override an existing key

grades = {"guy": 50}
grades["guy"] = 100
print(grades)

6. Deleting items

6.1 `del`

grades = {"guy": 50, "dylan": 80}
del grades["guy"]
print(grades)

6.2 `.pop(key)`

.pop() returns the removed value.

grades = {"guy": 50, "dylan": 80}
x = grades.pop("guy")
print(x)
print(grades)

6.3 `.clear()`

grades = {"guy": 50, "dylan": 80}
grades.clear()
print(grades)

7. Dictionary views: `.keys()`, `.values()`, `.items()`

grades = {"guy": 50, "dylan": 80, "rotem": 100}

print(grades.keys())
print(grades.values())
print(grades.items())

keys() returns a view of keys
values() returns a view of values
items() returns key-value pairs as tuples like: (key, value)

8. `for` loops with dictionaries (every common case)

8.1 Loop over keys (default)

grades = {"guy": 50, "dylan": 80, "rotem": 100}

for key in grades:
    print(key)

Same as:

for key in grades.keys():
    print(key)

8.2 Loop over values

grades = {"guy": 50, "dylan": 80, "rotem": 100}

for value in grades.values():
    print(value)

8.3 Loop over items (tuple pairs)

grades = {"guy": 50, "dylan": 80, "rotem": 100}

for item in grades.items():
    print(item)
    print(item[0], item[1])

8.4 Loop over items (recommended: unpacking)

grades = {"guy": 50, "dylan": 80, "rotem": 100}

for key, value in grades.items():
    print(f"the key is {key} and the value is {value}")

8.5 Using `continue` inside dict loops

grades = {"guy": 50, "dylan": 80, "rotem": 100}

for key, value in grades.items():
    if key == "rotem":
        continue
    print(value)

9. Membership checks

9.1 Check if a key exists

grades = {"guy": 50, "dylan": 80}

if "guy" in grades:
    print("exists")

9.2 Check if a value exists

grades = {"guy": 50, "dylan": 80}

if 80 in grades.values():
    print("found 80")

10. Common DevOps pattern: list of dictionaries (inventory)

This is a very common shape:

infrastructure = [
    {"id": "i-01", "status": "running", "type": "t2.micro", "os": "amazon linux"},
    {"id": "i-02", "status": "running", "type": "m5.large", "os": "amazon linux"},
    {"id": "i-03", "status": "stopped", "type": "r6.large", "os": "ubuntu"},
]

10.1 Loop and check status

for instance in infrastructure:
    if instance["status"] == "running":
        print(f"{instance['id']} is up and running")
    else:
        print(f"{instance['id']} is NOT running")

11. Merging dictionaries (tags use-case)

Example from class:

global_tags = {"env": "e2e", "owner": "all", "project": "AIdev"}
s3_tags = {"env": "e2e", "owner": "devops", "parent_asset_id": "46128461"}

Requirement:

In case of conflict, take the value from s3_tags.

11.1 Merge using loops

merged_tags = {}

for key in global_tags:
    merged_tags[key] = global_tags[key]

for key in s3_tags:
    merged_tags[key] = s3_tags[key]

print(merged_tags)

11.2 Merge with `.copy()` + `.update()`

merged_tags = global_tags.copy()
merged_tags.update(s3_tags)
print(merged_tags)

11.3 Merge with `|` (Python 3.9+)

merged_tags = global_tags | s3_tags
print(merged_tags)

12. Summing values (metrics use-case)

Example from class:

cpu_usage = {
    "service_auth": 129,
    "service_validate": 60,
    "service_payment": 122
}

12.1 Sum using a loop

total = 0
for key in cpu_usage:
    total += cpu_usage[key]
print(total)

12.2 Sum using `.values()`

print(sum(cpu_usage.values()))

13. Build a dict of squares (practice pattern)

Goal: keys are numbers, values are squares.

import random

dict1 = {}
iterations = 0

while True:
    r = input("enter a number (1-15) : (q to exit) ")

    if r == "q":
        if len(dict1) >= 10:
            break
        print("need more items")
        continue

    if r == "":
        r = random.randrange(1, 15)
    else:
        r = int(r)

    if r < 1 or r > 15:
        continue

    dict1[r] = r ** 2
    iterations += 1

print(dict1)
print(iterations)

14. Quick recap

Use d[key] when the key must exist.
Use d.get(key, default) when missing keys are normal.
Loop patterns:
- for k in d: keys
- for v in d.values(): values
- for k, v in d.items(): pairs
Use .update() or | to merge dictionaries.

Dictionaries (dict) + Looping Patterns for DevOps

1. Why dict is important