str Fundamentals¶

The str type represents text in Python.

A string is a sequence of Unicode characters. Strings are used for:

• words and sentences
• names and labels
• file paths
• source code text
• user input
• formatted output

Examples:

"hello"
"Python"
"123"
""
````

```mermaid
flowchart TD
    A[str]
    A --> B[text data]
    A --> C[sequence of characters]
    A --> D[immutable object]

1. Strings as Sequences¶

A string is not just a block of text. It is an ordered sequence of characters.

word = "Python"

This string contains six characters:

flowchart LR
    A["P"] --> B["y"] --> C["t"] --> D["h"] --> E["o"] --> F["n"]

Because strings are sequences, they support indexing, slicing, iteration, and membership testing.

2. Strings Are Immutable¶

Strings are immutable, which means their contents cannot be changed in place.

Example:

name = "Alice"

You cannot replace one character directly:

# name[0] = "M"   # TypeError

Instead, string operations create new strings.

new_name = "M" + name[1:]
print(new_name)

Output:

Mlice

3. Unicode Text¶

Python strings use Unicode, which allows them to represent text from many languages and symbol systems.

print("café")
print("你好")
print("😊")

This is one reason Python strings are more expressive than simple ASCII-only text models.

4. String Length¶

The number of characters in a string can be measured with len().

text = "Python"
print(len(text))

Output:

6

5. Empty Strings¶

An empty string contains no characters.

empty = ""
print(len(empty))

Output:

0

Empty strings are falsy in Boolean contexts.

if empty:
    print("non-empty")
else:
    print("empty")

6. Worked Examples¶

Example 1: basic string¶

language = "Python"
print(language)

Example 2: length¶

word = "code"
print(len(word))

Output:

4

Example 3: immutability idea¶

s = "cat"
t = "b" + s[1:]
print(t)

Output:

bat

7. Summary¶

Key ideas:

• str represents text
• strings are ordered sequences of Unicode characters
• strings are immutable
• strings can be empty or non-empty
• sequence operations are central to working with text

The str type is one of the most important and frequently used types in Python.