Table of Contents
Preparing narration.
Printing a value is easy. Printing it in the exact shape a person expects takes a little more thought. A price may need two decimal places, a percentage may need a percent sign, and a report line may need every column to line up.
String formatting separates the message from the rule for displaying each value. Python gives you a few styles, but str.format() and f-strings cover most code you will write today.
Using str.format()
The str.format() method replaces placeholders inside a string. A format specification after the colon controls the output:
Python
txt = "For only {price:.2f} dollars!"
print(txt.format(price = 49))Here price is inserted with two digits after the decimal point, so the output is For only 49.00 dollars!. The original number remains an integer; only its text representation changes.
The braces can contain a position or a name. Positional placeholders start at zero:
Python
message = "{} scored {} points."
print(message.format("Maya", 42))Named placeholders make a template easier to read when it has several values:
Python
message = "{name} has {count} messages."
print(message.format(name="Maya", count=42))The values do not need to be strings. Python converts them while building the final text, and the format specification tells it how to display a value. Named fields are especially useful when the same template is stored in one place and values arrive later.
You can also align text and numbers:
Python
rows = [("Tea", 3), ("Coffee", 12)]
for name, count in rows:
print("{:<10} {:>3}".format(name, count))The output is:
Text
Tea 3
Coffee 12< aligns to the left and > aligns to the right. The 10 and 3 are field widths, not limits on the values. A long string can still spill past its field.
Writing f-strings
An f-string puts the letter f before the opening quote. Expressions inside {} are evaluated when Python creates the string:
Python
name = "Tushar"
age = 23
print(f"Hello, my name is {name} and I am {age} years old.")The output is:
Text
Hello, my name is Tushar and I am 23 years old.You can put an expression inside the braces, not just a variable:
Python
items = 4
price = 12.5
print(f"Total: {items * price:.2f}")This prints:
Text
Total: 50.00The part after the colon is a format specification. .2f asks for a floating-point value with two digits after the decimal point. Python evaluates items * price first, then applies .2f to the result.
The same syntax handles percentages, alignment, and thousands separators:
Python
completion = 0.875
total = 1250000
print(f"Completion: {completion:.1%}")
print(f"Total: {total:,}")This prints Completion: 87.5% and Total: 1,250,000. The stored values are still 0.875 and 1250000; formatting changes what the reader sees.
If you need a literal brace, double it:
Python
name = "Maya"
print(f"{{user}}: {name}")The output is {user}: Maya. A single unmatched brace causes a SyntaxError, which is an annoying failure when a message is assembled from several pieces, so keep complicated templates readable.
Which style should you use
F-strings are usually easier to read when the values are already in scope. str.format() is still useful when the template is stored separately or when you want to pass named values explicitly.
Older code may use percent formatting, such as "Hello, %s" % name. It still works, but several values and format rules become harder to read. When you control the Python version, reach for an f-string first, then use str.format() when the template and values need to stay separate.
There is a safety boundary here. An f-string evaluates its Python expressions immediately, and formatting does not validate a value or escape it for HTML, SQL, or a shell command. Use the output library's escaping or parameter handling for those contexts. A nicely formatted string can still be unsafe input.
My default is f-strings for local variables and calculations, str.format() for reusable templates, and neither one as a substitute for validation. Once you know whether you are changing the value or only changing its display, the syntax becomes much less mysterious.
