Format Specifiers¶

Python's format specification mini-language controls how values are displayed in:

• f-strings
• str.format()
• the built-in format() function

The basic syntax is:

{value:format_spec}

Example:

python value = 42.5 print(f"{value:.1f}") # 42.5

Basic Alignment¶

Format specifiers can align text within a specified width.

```python text = "test"

print(f"{text:<10}") # 'test ' print(f"{text:>10}") # ' test' print(f"{text:^10}") # ' test ' ```

These correspond to the string alignment methods from the previous chapter:

Fill Characters¶

A fill character may be placed before the alignment symbol.

python print(f"{'test':*<10}") # test****** print(f"{'test':->10}") # ------test print(f"{'test':_^10}") # ___test___

The = alignment places padding between the sign and the digits.

python print(f"{-42:0=8}") # -0000042 print(f"{42:0=+8}") # +0000042

Width¶

Width specifies the minimum field size.

python for word in ["a", "abc", "abcdefgh"]: print(f"[{word:5}]")

Output:

[a ] [abc ] [abcdefgh]

Values longer than the width are not truncated.

Default alignment depends on the value type:

```python

Strings: left-aligned¶

print(f"{'abc':10}") # 'abc '

Numbers: right-aligned¶

print(f"{123:10}") # ' 123' ```

Precision¶

Precision controls decimal places for floats or maximum length for strings. Precision rounds floating-point values.

Float Precision¶

```python pi = 3.14159265

print(f"{pi:.0f}") # 3 print(f"{pi:.2f}") # 3.14 print(f"{pi:.4f}") # 3.1416 ```

String Precision¶

```python text = "Hello, World!"

print(f"{text:.5}") # Hello print(f"{text:10.5}") # Hello ```

Numeric Formatting¶

Numbers support several formatting options.

Sign Control¶

python print(f"{42:+}") # +42 print(f"{-42:+}") # -42

Thousands Separators¶

```python big = 1234567890

print(f"{big:,}") # 1,234,567,890 print(f"{big:_}") # 1_234_567_890 ```

Integer Bases¶

```python num = 255

print(f"{num:b}") # binary print(f"{num:x}") # hexadecimal print(f"{num:o}") # octal ```

With prefixes:

python print(f"{num:#b}") # 0b11111111 print(f"{num:#X}") # 0xFF

Floating-Point Types¶

Fixed Point¶

```python pi = 3.14159265

print(f"{pi:f}") # 3.141593 print(f"{pi:.2f}") # 3.14 print(f"{pi:10.2f}") # 3.14 ```

Scientific Notation¶

```python large = 1234567.89

print(f"{large:e}") # 1.234568e+06 print(f"{large:.2e}") # 1.23e+06 print(f"{large:.2E}") # 1.23E+06 ```

Percentage¶

```python ratio = 0.8567

print(f"{ratio:%}") # 85.670000% print(f"{ratio:.1%}") # 85.7% print(f"{ratio:.0%}") # 86% ```

Datetime Formatting¶

Datetime objects support strftime codes inside format specifiers. These codes are part of the datetime.strftime syntax, not the format mini-language itself.

```python from datetime import datetime

dt = datetime(2025, 3, 15)

print(f"{dt:%Y-%m-%d}") # 2025-03-15 print(f"{dt:%B %d, %Y}") # March 15, 2025 print(f"{dt:%A}") # Saturday ```

Time components:

```python dt = datetime(2025, 1, 12, 14, 30, 45)

print(f"{dt:%H:%M:%S}") # 14:30:45 print(f"{dt:%I:%M %p}") # 02:30 PM ```

Common Patterns¶

Currency Formatting¶

```python price = 1234.5

print(f"${price:,.2f}") # \(1,234.50 print(f"\)") # $1,234.50 ```

Table Alignment¶

```python data = [("Alice", 95.5), ("Bob", 87.3)]

for name, score in data: print(f"{name:<10} {score:>6.1f}") ```

Output:

Alice 95.5 Bob 87.3

ID Generation¶

python for i in range(1, 4): print(f"ID-{i:04d}")

Output:

ID-0001 ID-0002 ID-0003

Full Format Specification Grammar¶

The complete format specification pattern is:

[[fill]align][sign][#][0][width][,][.precision][type]

Example combining multiple components:

```python value = 42.5

print(f"{value:*>+10,.1f}")

****+42.5¶

```

Key Takeaways¶

• Format specifiers control how values are displayed.
• The syntax is {value:format_spec}.
• Alignment uses <, >, and ^.
• Width defines minimum field size.
• Precision controls decimal places or string length.
• Numeric types support bases, separators, and signs.
• Datetime formatting uses strftime codes.
• Format specifiers are commonly used in tables, reports, and numeric displays.

Exercises¶

Exercise 1. Format the number 1234567.891 as: (a) a float with 2 decimal places and comma separators, (b) scientific notation with 3 decimal places, and (c) a percentage (treating it as a ratio).

Exercise 2. Create a formatted table of 4 items showing a left-aligned name (20 chars), right-aligned price (10 chars, 2 decimal places), and centered status (10 chars). Use f-string format specifiers.

Exercise 3. Use the # prefix to display the number 255 in binary, octal, and hexadecimal with their respective prefixes (0b, 0o, 0x).