Strings — Deep Dive

স্ট্রিং — immutable sequence-এর গভীর বিশ্লেষণ

Read: ~35 min Intermediate 5 practice problems Live code runner

1. What a String Really Is

In Python, a string (str) is an immutable sequence of Unicode code points. "Immutable" means once a string exists, you cannot change it — every operation that looks like a change actually produces a new string.

Python-এ str হলো একটি immutable Unicode sequence — একবার তৈরি হলে আর পরিবর্তন করা যায় না। "পরিবর্তন"-এর মতো দেখালেও প্রতিটি operation আসলে একটি নতুন string তৈরি করে। এই immutability-ই Python-কে নিরাপদ ও হ্যাশযোগ্য করে তোলে।

2. Creating Strings

strings.py

single = 'hello'
double = "world"
multi  = """This is
a multi-line
string."""
bangla = "বাংলাদেশ"
raw    = r"C:\Users\abcl"        # backslashes literal
fstr   = f"Length = {len(bangla)}"   # f-string

print(single, double)
print(multi)
print(bangla, len(bangla))
print(raw)
print(fstr)

Single/double quote একরকম কাজ করে। """...""" multi-line-এর জন্য। r"..." raw string — backslash-কে literal রাখে (regex ও Windows path-এ কাজে লাগে)। f"..." expression-কে সরাসরি embed করে।

3. Indexing & Slicing — `s[a:b:c]`

Strings are sequences, so you can index them starting from 0, and use slices to extract sub-strings.

Figure 13.1 — Indexing "Python" from both directions.

slicing.py

s = "Python Programming"

print(s[0])        # P
print(s[-1])       # g  (last char)
print(s[0:6])      # Python
print(s[7:])       # Programming
print(s[:6])       # Python
print(s[::2])      # Pto rgamn  (every 2nd char)
print(s[::-1])     # reversed

# length
print(len(s))      # 18

4. The 30+ String Methods You Actually Use

Python's str has over 40 methods. Here are the essentials every programmer uses daily.

Method	Purpose	Example
`.lower() .upper() .title()`	Change case	`"AbC".lower() → "abc"`
`.strip() .lstrip() .rstrip()`	Trim whitespace	`" hi ".strip() → "hi"`
`.split(sep)`	Split into list	`"a,b,c".split(",") → ["a","b","c"]`
`.join(iterable)`	Join with separator	`",".join(["a","b"]) → "a,b"`
`.replace(old, new)`	Replace all occurrences	`"aXb".replace("X","Y") → "aYb"`
`.startswith(p) / .endswith(s)`	Prefix / suffix check	`"file.py".endswith(".py") → True`
`.find(sub) / .index(sub)`	Search. find → -1, index → error	`"banana".find("na") → 2`
`.count(sub)`	Count occurrences	`"banana".count("a") → 3`
`.isdigit() .isalpha() .isalnum()`	Character-class checks	`"123".isdigit() → True`
`.zfill(n) .ljust(n) .rjust(n)`	Pad	`"7".zfill(3) → "007"`
`.format() / f"..."`	Format strings	`f"{x:.2f}"`

methods.py

raw = "   Hello, Python World!   "

print(raw.strip())
print(raw.strip().lower())
print(raw.strip().replace("Python", "Bangla"))
print(raw.strip().split(", "))

# build a sentence
parts = ["ABCL", "TECH", "Courses"]
print(" · ".join(parts))

# format
price = 1299.5
print(f"Price: {price:,.2f} BDT")

5. Immutability — And Why It Matters

s[0] = "X" raises TypeError. String methods never modify in place — they return new strings. This makes strings hashable (so they can be dict keys or set elements) and safe to share.

immutable.py

s = "cat"
print(id(s), s)

s = s.upper()            # creates a NEW string
print(id(s), s)    # different id — different object

# assignment rebinds the name; old "cat" is garbage-collected

# hashable, so can be a dict key
d = {"cat": 1, "dog": 2}
print(d["cat"])

6. Encoding — `str` vs `bytes`, UTF-8 and Bangla

A Python str is Unicode (code points). To store or transmit, you need bytes in some encoding — UTF-8 is the de-facto standard. Bangla characters use 3 bytes each in UTF-8.

encoding.py

text = "বাংলা"
print(len(text))                       # 5  (characters)

encoded = text.encode("utf-8")
print(encoded)                         # b'\xe0\xa6\xac...'
print(len(encoded))                    # 15 (bytes — 3 per char)

decoded = encoded.decode("utf-8")
print(decoded == text)                  # True

len(text) অক্ষর গোনে, bytes গোনে না। ফাইল read/write-এর সময় সবসময় encoding="utf-8" দিন — Bangla, Emoji, চাইনিজ — সব নিরাপদে হ্যান্ডেল হবে।

7. Vocabulary

Term	Meaning	বাংলায়
Immutable	Cannot be changed after creation.	তৈরির পর পরিবর্তন করা যায় না।
Slice	Sub-sequence via `[a:b:c]`.	`[a:b:c]` দিয়ে নেওয়া অংশ।
Code point	A Unicode character value.	একটি Unicode অক্ষর।
Encoding	Code points → bytes.	Code point-কে bytes-এ রূপান্তর।
UTF-8	Variable-length encoding, dominant on the web.	পরিবর্তনশীল দৈর্ঘ্যের encoding।
Raw string	`r"..."` — backslashes are literal.	`r"..."` — backslash literal।

8. Practice Problems

Reverse a string without using [::-1].

[::-1] ছাড়া একটি string উল্টান।
✨ Show Answer
ans1.py
```
def reverse(s):
    out = ""
    for ch in s:
        out = ch + out
    return out

print(reverse("Python"))
```
Check whether a string is a palindrome (ignoring case).

একটি string palindrome কিনা check করুন (case ignore করে)।
✨ Show Answer
ans2.py
```
def is_palindrome(s):
    s = s.lower()
    return s == s[::-1]

print(is_palindrome("Level"))
print(is_palindrome("Python"))
```
Count the number of vowels in a sentence.

একটি বাক্যে vowel-এর সংখ্যা গণনা করুন।
✨ Show Answer
ans3.py
```
sentence = "Learning Python is wonderful"
vowels = "aeiouAEIOU"
count = sum(1 for c in sentence if c in vowels)
print(f"Vowels: {count}")
```
Why can a string be a dict key but a list cannot?

কেন string dict-এর key হতে পারে কিন্তু list পারে না?

✨ Show Answer

Dict keys must be hashable. Hashability requires the object's hash value to never change during its lifetime — which in turn requires immutability. Strings are immutable, so their hash is stable. Lists are mutable, so their hash could change after insertion and break the dict.

Dict-এর key অবশ্যই hashable হতে হবে। Hash মান object-এর পুরো জীবনকালে একই থাকতে হবে, যা immutability দাবি করে। String immutable, তাই hash স্থির। List mutable — hash বদলাতে পারে, তাই key হিসেবে ব্যবহার নিষিদ্ধ।

Given "2025-04-17", split into year/month/day and print "17 April 2025".

"2025-04-17" থেকে year/month/day বের করে "17 April 2025" প্রিন্ট করুন।

✨ Show Answer

ans5.py

months = ["January","February","March","April","May","June",
          "July","August","September","October","November","December"]

date = "2025-04-17"
y, m, d = date.split("-")
print(f"{int(d)} {months[int(m)-1]} {y}")

Summary — Module 13

A Python string is an immutable sequence of Unicode characters. Index with s[i], slice with s[a:b:c], and transform with the huge family of built-in methods. Strings never change in place — methods return new strings. Encode to bytes (UTF-8 preferred) when talking to files, sockets, or APIs.

Python-এর string — Unicode character-এর immutable sequence। s[i] দিয়ে index, s[a:b:c] দিয়ে slice। Method-গুলো নতুন string return করে, original অপরিবর্তিত থাকে। File/network-এ লেখার সময় UTF-8-এ encode করুন।

Next Module → Lists — Python-এর সবচেয়ে ব্যবহৃত mutable sequence।

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