Learn String Immutability

In Python, strings are immutable: once created, the characters they contain cannot be changed in place. Any "modification" you do actually creates a new string. This matters for correctness (no accidental in-place edits) and for performance (how you build larger strings).

No In-Place Edits

You can read characters by index, but you cannot assign to them.


              123456
            
s = "hello"

t = "H" + s[1:]   # Creates a new string: "Hello"
print(t)

s[0] = "H"      # TypeError: strings don't support item assignment

Most string methods return a new string and leave the original unchanged.


              123456
            
name = "  Alice  "
name.strip()            # returns "Alice" but doesn't change 'name'
print(name)             # "  Alice  "

name = name.strip()     # assign the new value
print(name)             # "Alice"

Chaining is fine, but remember you're getting a new object each step.


              1234
            
msg = "  hello\n"
clean = msg.strip().upper()   
print(msg)                    # original remains "  hello\n"
print(clean)                  # "HELLO"

"Modifying" by creating a new string

Use slicing, replace, or concatenation to produce a new value.


              1234567
            
s = "data"

s = s.replace("t", "T")       # "daTa"
print(t)

s = s[:1] + "A" + s[2:]       # "dAta"
print(t)

Efficient building

Repeated + in large loops can be slow (many intermediate strings). A common pattern is to collect pieces and join once:


              12
            
parts = ["ID:", " ", "123", "\n", "OK"]
result = "".join(parts)       # "ID: 123\nOK"

Note

You'll learn more joining/formatting patterns in the next chapter.

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Section 3. Chapter 4

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No In-Place Edits

You can read characters by index, but you cannot assign to them.


              123456
            
s = "hello"

t = "H" + s[1:]   # Creates a new string: "Hello"
print(t)

s[0] = "H"      # TypeError: strings don't support item assignment

Most string methods return a new string and leave the original unchanged.


              123456
            
name = "  Alice  "
name.strip()            # returns "Alice" but doesn't change 'name'
print(name)             # "  Alice  "

name = name.strip()     # assign the new value
print(name)             # "Alice"

Chaining is fine, but remember you're getting a new object each step.


              1234
            
msg = "  hello\n"
clean = msg.strip().upper()   
print(msg)                    # original remains "  hello\n"
print(clean)                  # "HELLO"

"Modifying" by creating a new string

Use slicing, replace, or concatenation to produce a new value.


              1234567
            
s = "data"

s = s.replace("t", "T")       # "daTa"
print(t)

s = s[:1] + "A" + s[2:]       # "dAta"
print(t)

Efficient building

Repeated + in large loops can be slow (many intermediate strings). A common pattern is to collect pieces and join once:


              12
            
parts = ["ID:", " ", "123", "\n", "OK"]
result = "".join(parts)       # "ID: 123\nOK"

Note

You'll learn more joining/formatting patterns in the next chapter.

Everything was clear?

Thanks for your feedback!

Section 3. Chapter 4

String Immutability

No In-Place Edits

"Modifying" by creating a new string

Efficient building

1. Which line attempts to modify a string in place and will raise an error?

2. What will the code output?

3. You need to assemble a long string from many small pieces. What's recommended?

Awesome!

String Immutability

No In-Place Edits

"Modifying" by creating a new string

Efficient building

1. Which line attempts to modify a string in place and will raise an error?

2. What will the code output?

3. You need to assemble a long string from many small pieces. What's recommended?