Formatting Strings with Python

Python
# Format spec mini-language
f"{value:>10.2f}"          # right-align, 2 decimals
f"{value:,}"                # thousands separator
f"{value:.1%}"               # percentage

# string.Template for user-facing substitution (safer than % or .format for untrusted input)
from string import Template
t = Template("Hello, $name! You have $count new messages.")
t.substitute(name="Ana", count=3)
t.safe_substitute(name="Ana")   # missing keys left as-is instead of raising

# Useful constants
import string
string.ascii_letters   # 'abcdefghij...XYZ'
string.digits          # '0123456789'
string.punctuation     # '!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~'

Progress:

• Step 1: Decide the right tool — f-string (static, dev-controlled), str.format() (reusable template, dev-controlled), or string.Template (untrusted/user-supplied templates)
• Step 2: Choose format spec ({:spec}) for numeric/alignment/width needs
• Step 3: For repeated custom formatting logic, subclass string.Formatter and override format_field/get_value
• Step 4: For generating random strings/tokens, combine string.ascii_letters, digits, punctuation with random/secrets
• Step 5: Test edge cases — missing keys, special characters, locale-sensitive numbers

Format Spec Mini-Language Reference

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

align: < > ^ =        (left, right, center, pad-after-sign)
sign:  + - ' '         (always show, only negative, space for positive)
type:  d f e g % x o b n s

Common patterns:

Python
f"{n:04d}"        # zero-padded int, width 4      -> "0007"
f"{n:+d}"         # force sign                     -> "+7"
f"{pi:.3f}"       # 3 decimal places                -> "3.142"
f"{big:,}"        # comma thousands separator       -> "1,234,567"
f"{ratio:.2%}"    # percentage with 2 decimals      -> "45.67%"
f"{text:^20}"     # center in width 20
f"{n:#x}"         # hex with 0x prefix

Example 1: Safe template for user-editable messages Input: App lets users customize a notification template like "Hi $user, your order $order_id shipped." Output:

Python
from string import Template

template = Template(user_supplied_template)
message = template.safe_substitute(user="Ana", order_id="A1023")
# safe_substitute avoids KeyError crashes if a $variable is missing/mistyped

Example 2: Custom Formatter for domain objects Input: Need {obj:currency} to format a Decimal as $1,234.50. Output:

Python
from string import Formatter
from decimal import Decimal

class MyFormatter(Formatter):
    def format_field(self, value, spec):
        if spec == "currency":
            return f"${value:,.2f}"
        return super().format_field(value, spec)

fmt = MyFormatter()
fmt.format("Total: {0:currency}", Decimal("1234.5"))
# -> "Total: $1,234.50"

Example 3: Generating a random alphanumeric token Input: Need an 8-char token from letters+digits. Output:

Python
import secrets, string

alphabet = string.ascii_letters + string.digits
token = "".join(secrets.choice(alphabet) for _ in range(8))