String Methods, Conditional Statements (if/elif/else), Loops (for, range())

🐍 Python for DevOps — Class 5 Notes

Topic: String Methods, Conditional Statements (`if/elif/else`), Loops (`for`, `range()`)

📌 Key Concepts Overview

Concept	One-Line Definition
String Methods	Built-in functions to inspect and transform strings
`if / elif / else`	Execute code blocks based on conditions
`for` loop	Iterate over any iterable — string, list, range, dict
`range()`	Generate a sequence of numbers to loop over
Nested `if`	An `if` block inside another `if` block
Nested `for`	A `for` loop inside another `for` loop

🔤 Part 1 — String Methods (Continuation from Class 4)

Checking String Case

# islower() — True if ALL characters are lowercase
'devops'.islower()     # True
'Devops'.islower()     # False

# isupper() — True if ALL characters are uppercase
'DEVOPS'.isupper()     # True
'Devops'.isupper()     # False

# Practical: normalize before comparing
env = input('Enter environment: ')
if env.lower() == 'production':
    print('⚠️ Deploying to PROD!')

Checking Digit / Numeric Strings

Method	Accepts	Rejects	DevOps Use
`isdecimal()`	`0-9` only	decimals, superscripts	Port validation
`isdigit()`	`0-9` + superscripts	decimal points	Version numbers
`isnumeric()`	`0-9` + superscripts + fractions	decimal points	Broadest check

# Key difference — decimal point breaks all three
'8080'.isdecimal()    # True
'8080'.isdigit()      # True
'8080'.isnumeric()    # True

'80.80'.isdecimal()   # False — dot is not a digit
'80.80'.isdigit()     # False
'80.80'.isnumeric()   # False

# DevOps use: validate user input before casting
port_input = input('Enter port: ')
if port_input.isdecimal():
    port = int(port_input)
else:
    print('❌ Invalid port — must be a whole number')

`replace()` — Find and Replace in Strings

# str.replace('old', 'new')
'Python'.replace('P', 'J')              # 'Jython'
'prod-server-01'.replace('-', '_')      # 'prod_server_01'
'This is python class'.replace('i','a') # 'Thas as python class' — ALL occurrences

# DevOps: sanitize strings for use in AWS resource names (no spaces allowed)
resource_name = 'my resource name'
safe_name = resource_name.replace(' ', '-')  # 'my-resource-name'

# Remove spaces entirely
'Pyt hon'.replace(' ', '')    # 'Python'

`startswith()` / `endswith()`

log_line = 'ERROR: Connection refused on port 5432'

# Route logs based on prefix
if log_line.startswith('ERROR'):
    print('🚨 Alert triggered')
elif log_line.startswith('WARN'):
    print('⚠️ Warning logged')

# Check file types
filename = 'deploy.sh'
if filename.endswith('.sh'):
    print('Shell script — set execute permission')
elif filename.endswith('.py'):
    print('Python script')
elif filename.endswith('.tf'):
    print('Terraform file')

🔀 Part 2 — Conditional Statements

`if` — Single Condition

# if condition:
#     block runs only when condition is True
# (anything outside the block always runs)

a = 392
if a % 2 == 0:
    print('Even')        # runs only if condition True

print('Done')            # ALWAYS runs — outside if block

# ⚠️ Common mistake from class:
# Putting "Number is odd" OUTSIDE the if block
# It prints even when number is even — logic bug

`if-else` — Two Branches

# if condition:
#     runs when True
# else:
#     runs when False

a = int(input('Enter a number: '))
if a % 2 == 0:
    print('Even')
else:
    print('Odd')

# DevOps: health check routing
status_code = 200
if status_code == 200:
    print('✅ Service healthy')
else:
    print(f'❌ Service unhealthy — code: {status_code}')

Multiple `if` vs `if-elif-else` — Critical Difference

# ❌ Multiple if — checks ALL conditions independently (inefficient)
a = 1
if a == 1: print('One')      # matches — prints One
if a == 2: print('Two')      # checks anyway — no match
if a == 3: print('Three')    # checks anyway — no match
# All 5 conditions are evaluated even after a match

# ✅ if-elif-else — stops at FIRST match (efficient)
if a == 1:
    print('One')             # matches — STOPS here
elif a == 2:
    print('Two')             # SKIPPED
elif a == 3:
    print('Three')           # SKIPPED
else:
    print('Out of range')    # SKIPPED

Rule of thumb: Use elif when conditions are mutually exclusive (only one can be true). Use multiple if when you need to check all conditions independently.

`if-elif-else` — Full Pattern

# HTTP status code handler (real DevOps pattern)
status = int(input('Enter HTTP status code: '))

if status == 200:
    print('OK — Success')
elif status == 201:
    print('Created — Resource provisioned')
elif status == 400:
    print('Bad Request — Check input params')
elif status == 401:
    print('Unauthorized — Check IAM/credentials')
elif status == 403:
    print('Forbidden — Check permissions')
elif status == 404:
    print('Not Found — Resource missing')
elif status == 500:
    print('Internal Server Error — Check app logs')
else:
    print(f'Unknown status: {status}')

`in` Operator with Conditionals

# Check membership — works on strings, lists, tuples, sets, dicts
x = input('Enter a letter: ')

if x.lower() in 'aeiou':
    print('Vowel')
else:
    print('Consonant')

# DevOps: check if region is valid
region = input('Enter AWS region: ')
valid_regions = ['ap-south-1', 'us-east-1', 'eu-west-1']
if region in valid_regions:
    print('✅ Valid region')
else:
    print('❌ Invalid region')

Nested `if-else`

# When you need to check conditions inside conditions
# Class used: pass Python AND Linux AND AWS to clear exam

p = int(input('Python score: '))
l = int(input('Linux score: '))
a = int(input('AWS score: '))

if p > 75:
    if l > 75:
        if a > 75:
            print('✅ Cleared all subjects')
        else:
            print('❌ Work hard in AWS')
    else:
        print('❌ Work hard in Linux')
else:
    print('❌ Work hard in Python')

# DevOps equivalent: multi-condition deployment gate
cpu = 45
memory = 70
disk = 60

if cpu < 80:
    if memory < 85:
        if disk < 90:
            print('✅ All checks passed — safe to deploy')
        else:
            print('❌ Disk usage too high')
    else:
        print('❌ Memory usage too high')
else:
    print('❌ CPU usage too high')

🔁 Part 3 — Loops

`for` Loop — Iterating Over Iterables

# Syntax: for variable in iterable:
#             block

# Iterables: string, list, tuple, set, dict, range, bytes

# Over a string — character by character
for char in 'Python':
    print(char)     # P y t h o n (each on new line)

# Over a list
servers = ['web-01', 'db-01', 'cache-01']
for server in servers:
    print(f'Pinging {server}...')

# Over a list with condition
for server in servers:
    if server.startswith('db'):
        print(f'⚠️ Database server: {server}')

`range()` — Generating Number Sequences

# range(stop)          → 0 to stop-1
# range(start, stop)   → start to stop-1
# range(start, stop, step) → with step size

range(10)           # 0, 1, 2, ..., 9
range(1, 11)        # 1, 2, ..., 10
range(0, 51, 2)     # 0, 2, 4, ..., 50  (even numbers)
range(-1, -7, -1)   # -1, -2, ..., -6  (reverse traversal)

# Common patterns
for i in range(10):
    print('DevOps')     # print 10 times

for i in range(1, 51):
    if i % 2 == 0:
        print(i)        # even numbers 2-50

Looping with Index — `range(len())`

# When you need BOTH the index and the value
str1 = 'Python'
for i in range(len(str1)):
    print(i, str1[i])
# 0 P
# 1 y
# 2 t
# 3 h
# 4 o
# 5 n

# Find all positions of a character
log = 'ERROR: disk full. ERROR: retry failed.'
for i in range(len(log)):
    if log[i] == 'E' and log[i:i+5] == 'ERROR':
        print(f'ERROR found at index {i}')

String + Loop Patterns

# Extract vowels from a string
str1 = 'wqciecneucqbcjeqbviuqciwqnkeqnv'
for char in str1:
    if char.upper() in 'AEIOU':
        print(char)

# Get all uppercase letters
str1 = 'This is Python Class.'
for char in str1:
    if char.isupper():
        print(char)    # T, P, C

# Reverse a string using loop
str1 = 'Python'
rev_str = ''
for i in range(-1, -len(str1)-1, -1):
    rev_str += str1[i]
print(rev_str)         # nohtyP

Nested `for` Loops

# Loop inside a loop — inner loop completes FULLY for each outer iteration

# Class example: print each word AND each character
lst = ['Peter', 'piper', 'picked']

for word in lst:
    print(word)
    for char in word:
        print(char)

# DevOps: scan all servers for all ports
servers = ['web-01', 'db-01']
ports = [22, 80, 443]

for server in servers:
    for port in ports:
        print(f'Checking {server}:{port}')
# web-01:22, web-01:80, web-01:443, db-01:22 ...

☁️ DevOps / Cloud Use Cases

# 1. Log line parser — classify log levels
logs = [
    'ERROR: DB connection failed',
    'INFO: Deployment started',
    'WARN: CPU at 82%',
    'ERROR: Pod CrashLoopBackOff',
]

for log in logs:
    if log.startswith('ERROR'):
        print(f'🚨 ALERT: {log}')
    elif log.startswith('WARN'):
        print(f'⚠️  WARN: {log}')
    else:
        print(f'ℹ️  INFO: {log}')

# 2. Port scanner stub (basic)
target_ports = [22, 80, 443, 3306, 5432, 8080, 8443]
for port in target_ports:
    if port in [22, 80, 443]:
        print(f'Port {port} — standard, expected open')
    elif port in [3306, 5432]:
        print(f'Port {port} — DB port, should be private!')
    else:
        print(f'Port {port} — verify if needed')

# 3. Multi-region health check
regions = ['ap-south-1', 'us-east-1', 'eu-west-1']
for region in regions:
    if region.startswith('ap'):
        print(f'{region} → APAC cluster')
    elif region.startswith('us'):
        print(f'{region} → US cluster')
    else:
        print(f'{region} → EU cluster')

# 4. Validate resource name before creating
name = input('Enter resource name: ')
safe_name = name.replace(' ', '-').lower()
if safe_name.replace('-', '').isalnum():
    print(f'✅ Valid name: {safe_name}')
else:
    print('❌ Name has invalid characters')

# 5. Find ERROR count in a log file (simulated)
log_data = 'INFO start\nERROR timeout\nINFO retry\nERROR disk full\nINFO done'
error_count = 0
for line in log_data.split('\n'):
    if line.startswith('ERROR'):
        error_count += 1
print(f'Total errors: {error_count}')

❌ Common Mistakes

Mistake	Code	Fix
Code outside `if` always runs	`print('Odd')` after the `if` block	Put it inside `else`
Multiple `if` instead of `elif`	All conditions check even after match	Use `elif` for exclusive branches
`range(10)` gives 0–9, not 1–10	`range(10)` → 0,1...9	`range(1, 11)` for 1 to 10
Off-by-one in reverse range	`range(-1, -6, -1)` for 6-char string	`range(-1, -len(s)-1, -1)`
`in` on wrong type	`if port in '8080'` → checks substring in string	`if port in [8080, 443]` — use a list
`islower()` on string with spaces	`'hello world'.islower()` → `True`	Spaces are ignored — only letters checked
Forgetting colon after `if`/`for`	`if x == 1` → SyntaxError	Always end with `:`
Wrong indentation	Mixed tabs/spaces	Use 4 spaces consistently

🎯 Interview Points

"Difference between if-elif and multiple if statements?"
→ elif stops after the first match — faster, mutually exclusive. Multiple if evaluates every condition every time.
"What is the difference between isdecimal(), isdigit(), isnumeric()?"
→ isdecimal() is strictest (0-9 only). isdigit() also accepts superscripts. isnumeric() is broadest — accepts fractions too. In DevOps, use isdecimal() for port/number validation.
"How does for loop work internally?"
→ Python calls iter() on the iterable, then repeatedly calls next() until StopIteration. Every for loop is really an iterator under the hood.
"What does range(1, 10) return?"
→ 1 through 9 — stop value is excluded. range(1, 11) for 1 through 10.
"When would you use nested loops in DevOps?"
→ Scanning multiple servers × multiple ports. Processing multi-dimensional data. Parsing nested config structures.
"Can you use in with a dict?"
→ in on a dict checks keys only, not values. 'key' in my_dict → checks keys.
"What is the in operator?"
→ Membership test — returns True if element exists in the iterable. Works on strings, lists, tuples, sets, dict keys.

📚 Knowledge Base — Quick Revision

# ── STRING METHODS ──────────────────────────────────────────
'devops'.islower()              # True
'DEVOPS'.isupper()              # True
'8080'.isdecimal()              # True  ← use for port validation
'8.0'.isdecimal()               # False ← dot breaks it
'hello'.replace('l', 'r')       # 'herro'
'error.log'.endswith('.log')    # True
'ERROR'.startswith('ERR')       # True

# ── CONDITIONALS ────────────────────────────────────────────
if condition:
    ...
elif condition:         # else-if — stops at first match
    ...
else:
    ...

# Use 'in' for membership
if value in [list, of, options]:
    ...

# ── FOR LOOP ────────────────────────────────────────────────
for item in iterable:       # iterates over elements
    ...

for i in range(10):         # 0 to 9
    ...

for i in range(1, 11):      # 1 to 10
    ...

for i in range(0, 10, 2):   # 0,2,4,6,8 (step=2)
    ...

for i in range(len(s)):     # index-based traversal
    print(i, s[i])

# ── NESTED LOOPS ────────────────────────────────────────────
for server in servers:
    for port in ports:
        print(f'{server}:{port}')

# ── STRING REVERSE ───────────────────────────────────────────
rev = ''
for i in range(-1, -len(s)-1, -1):
    rev += s[i]
# or simply: s[::-1]

🏋️ Practice Questions

Easy

Write a script that checks if a given string (like a resource name) contains only lowercase letters using islower(). If not, convert it and print the result.
Use a for loop with range() to print all even numbers from 1 to 100.
Take a filename as input (e.g., deploy.sh, main.tf, config.yml). Use endswith() to print what type of file it is.

Medium

Write a script that takes a port number string as input, validates it using isdecimal(), casts it to int, and checks if it falls in one of these ranges: Well-known (0–1023), Registered (1024–49151), Dynamic (49152–65535). Print the category.
Given a log string like 'INFO start\nERROR timeout\nWARN disk 80%\nERROR crash', loop through each line and count how many are ERROR, WARN, and INFO. Print the counts.
Write a script that builds a "safe" AWS resource name from user input: strip spaces with replace(), convert to lowercase, validate with isdecimal() / isalnum(), and print either the safe name or an error.

DevOps-Focused

Deployment Gate Script: Ask the user for CPU %, memory %, and disk % (all integers). Use nested if-else to decide: if all are under 80%, print "✅ Safe to deploy". If any one exceeds, print which resource is the bottleneck. Use f-strings for output.
Log Analyzer: Given this list of log lines:

   logs = ['ERROR: pod crash', 'INFO: deploy started', 'ERROR: OOM killed',
           'WARN: cpu spike', 'INFO: rollback complete', 'ERROR: disk full']

Loop through and: (a) print ERROR lines in uppercase using .upper(), (b) count total errors, (c) print a final summary: "Found X errors in Y total log lines".

推荐订阅源

DEV Community

🐍 Python for DevOps — Class 5 Notes

Topic: String Methods, Conditional Statements (`if/elif/else`), Loops (`for`, `range()`)

📌 Key Concepts Overview

🔤 Part 1 — String Methods (Continuation from Class 4)

Checking String Case

Checking Digit / Numeric Strings

`replace()` — Find and Replace in Strings

`startswith()` / `endswith()`

🔀 Part 2 — Conditional Statements

`if` — Single Condition

`if-else` — Two Branches

Multiple `if` vs `if-elif-else` — Critical Difference

`if-elif-else` — Full Pattern

`in` Operator with Conditionals

Nested `if-else`

🔁 Part 3 — Loops

`for` Loop — Iterating Over Iterables

`range()` — Generating Number Sequences

Looping with Index — `range(len())`

String + Loop Patterns

Nested `for` Loops

☁️ DevOps / Cloud Use Cases

❌ Common Mistakes

🎯 Interview Points

📚 Knowledge Base — Quick Revision

🏋️ Practice Questions

Easy

Medium

DevOps-Focused

推荐订阅源

DEV Community

🐍 Python for DevOps — Class 5 Notes

Topic: String Methods, Conditional Statements (if/elif/else), Loops (for, range())

📌 Key Concepts Overview

🔤 Part 1 — String Methods (Continuation from Class 4)

Checking String Case

Checking Digit / Numeric Strings

replace() — Find and Replace in Strings

startswith() / endswith()

🔀 Part 2 — Conditional Statements

if — Single Condition

if-else — Two Branches

Multiple if vs if-elif-else — Critical Difference

if-elif-else — Full Pattern

in Operator with Conditionals

Nested if-else

🔁 Part 3 — Loops

for Loop — Iterating Over Iterables

range() — Generating Number Sequences

Looping with Index — range(len())

String + Loop Patterns

Nested for Loops

☁️ DevOps / Cloud Use Cases

❌ Common Mistakes

🎯 Interview Points

📚 Knowledge Base — Quick Revision

🏋️ Practice Questions

Easy

Medium

DevOps-Focused

Topic: String Methods, Conditional Statements (`if/elif/else`), Loops (`for`, `range()`)

`replace()` — Find and Replace in Strings

`startswith()` / `endswith()`

`if` — Single Condition

`if-else` — Two Branches

Multiple `if` vs `if-elif-else` — Critical Difference

`if-elif-else` — Full Pattern

`in` Operator with Conditionals

Nested `if-else`

`for` Loop — Iterating Over Iterables

`range()` — Generating Number Sequences

Looping with Index — `range(len())`

Nested `for` Loops