209. Writing Custom Python Formatters

🔹 1. Basic Custom Formatter Using __format__

• We override the __format__ method in a class.

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class Currency:
    def __init__(self, amount):
        self.amount = amount

    def __format__(self, format_spec):
        return f"${self.amount:,.2f}" if format_spec == "currency" else str(self.amount)

price = Currency(12345.678)
print(f"Total: {price:currency}")  # Total: $12,345.68

🔍 How it works:

• If "currency" format is used, it returns a formatted string with $ and two decimal places.

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🔹 2. Formatting Dates in Custom Format

• We format a datetime object using a custom formatter.

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from datetime import datetime

class CustomDate:
    def __init__(self, dt):
        self.dt = dt

    def __format__(self, format_spec):
        formats = {
            "short": "%d-%m-%Y",
            "long": "%A, %d %B %Y",
        }
        return self.dt.strftime(formats.get(format_spec, "%Y-%m-%d"))

today = CustomDate(datetime(2024, 8, 25))
print(f"Date (short): {today:short}")  # Date (short): 25-08-2024
print(f"Date (long): {today:long}")  # Date (long): Sunday, 25 August 2024

🔍 How it works:

• Provides multiple date formats based on the format specifier.

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🔹 3. Custom Formatter for Percentage Representation

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class Percentage:
    def __init__(self, value):
        self.value = value

    def __format__(self, format_spec):
        return f"{self.value:.2%}" if format_spec == "percent" else str(self.value)

accuracy = Percentage(0.875)
print(f"Model Accuracy: {accuracy:percent}")  # Model Accuracy: 87.50%

🔍 How it works:

• The "percent" format multiplies the value by 100 and appends %.

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🔹 4. Custom Formatter for Hex, Binary, and Octal

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class Number:
    def __init__(self, value):
        self.value = value

    def __format__(self, format_spec):
        formats = {
            "hex": f"0x{self.value:x}",
            "bin": f"0b{self.value:b}",
            "oct": f"0o{self.value:o}",
        }
        return formats.get(format_spec, str(self.value))

num = Number(255)
print(f"Hex: {num:hex}")  # Hex: 0xff
print(f"Binary: {num:bin}")  # Binary: 0b11111111
print(f"Octal: {num:oct}")  # Octal: 0o377

🔍 How it works:

• Uses different format specifiers to convert numbers to hex, binary, and octal.

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🔹 5. Custom Formatter for Human-Readable File Sizes

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class FileSize:
    def __init__(self, size_in_bytes):
        self.size = size_in_bytes

    def __format__(self, format_spec):
        units = ["B", "KB", "MB", "GB", "TB"]
        size = self.size
        for unit in units:
            if size < 1024:
                return f"{size:.2f} {unit}"
            size /= 1024
        return f"{size:.2f} PB"

file = FileSize(1048576)
print(f"File Size: {file}")  # File Size: 1.00 MB

🔍 How it works:

• Automatically converts bytes into KB, MB, GB, etc. for readability.

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🔹 6. Formatting Phone Numbers

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class PhoneNumber:
    def __init__(self, number):
        self.number = number

    def __format__(self, format_spec):
        if format_spec == "us":
            return f"({self.number[:3]}) {self.number[3:6]}-{self.number[6:]}"
        return self.number

phone = PhoneNumber("9876543210")
print(f"Call me at {phone:us}")  # Call me at [phone]

🔍 How it works:

• Formats a phone number based on a country-specific format.

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🔹 7. Custom Formatter for Temperature Conversion

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class Temperature:
    def __init__(self, celsius):
        self.celsius = celsius

    def __format__(self, format_spec):
        if format_spec == "F":
            return f"{self.celsius * 9/5 + 32:.1f}°F"
        elif format_spec == "K":
            return f"{self.celsius + 273.15:.2f}K"
        return f"{self.celsius}°C"

temp = Temperature(25)
print(f"Temperature: {temp}")  # Temperature: 25°C
print(f"In Fahrenheit: {temp:F}")  # In Fahrenheit: 77.0°F
print(f"In Kelvin: {temp:K}")  # In Kelvin: 298.15K

🔍 How it works:

• Converts Celsius to Fahrenheit and Kelvin based on the format specifier.

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🔹 8. Custom Formatter for Masking Sensitive Data

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class MaskedString:
    def __init__(self, text):
        self.text = text

    def __format__(self, format_spec):
        return "*" * (len(self.text) - 4) + self.text[-4:] if format_spec == "mask" else self.text

card_number = MaskedString("1234567812345678")
print(f"Card Number: {card_number:mask}")  # Card Number: ************5678

🔍 How it works:

• Masks all characters except the last 4 for privacy.

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🔹 9. Custom Formatter for Text Alignment

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class AlignedText:
    def __init__(self, text):
        self.text = text

    def __format__(self, format_spec):
        if format_spec == "center":
            return self.text.center(30, "-")
        elif format_spec == "right":
            return self.text.rjust(30, "-")
        return self.text.ljust(30, "-")

text = AlignedText("Hello")
print(f"{text:center}")  # -----------Hello------------
print(f"{text:right}")   # ---------------------Hello
print(f"{text}")         # Hello---------------------

🔍 How it works:

• Aligns text left, center, or right using - as a padding character.

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🔹 10. Custom Formatter for JSON Pretty Printing

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import json

class JsonFormatter:
    def __init__(self, data):
        self.data = data

    def __format__(self, format_spec):
        return json.dumps(self.data, indent=4) if format_spec == "pretty" else json.dumps(self.data)

data = JsonFormatter({"name": "Alice", "age": 30, "city": "NY"})
print(f"Data:\n{data:pretty}")  # Pretty-printed JSON

🔍 How it works:

• Returns pretty-printed JSON when the "pretty" format is used.

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