F-Strings and String Formatting

Note

Source: This section replaces the C# edition’s writeline-substitution topic. F-string content is an original addition specific to Python 3.6+. The older str.format() and % formatting sections are derived from the SE4ML Python chapter (chapter_python.rst, lines 2554–2709).

Combining strings with variable values is one of the most common tasks in a program. Python’s f-strings (formatted string literals) are the modern, readable way to do it.

Basic F-Strings

An f-string is a string literal prefixed with f (or F). Inside the string, any expression enclosed in curly braces {} is evaluated and its value is inserted:

>>> name = "Alice"
>>> age = 20
>>> print(f"My name is {name} and I am {age} years old.")
My name is Alice and I am 20 years old.

Any Python expression can go inside {}:

>>> x = 5
>>> print(f"The square of {x} is {x ** 2}.")
The square of 5 is 25.

>>> a, b = 3, 4
>>> print(f"The hypotenuse is {(a**2 + b**2) ** 0.5:.2f}.")
The hypotenuse is 5.00.

Format Specifiers

After the expression you can add a format specifier separated by a colon to control how the value is displayed.

Floating-point precision:

>>> pi = 3.141592653589793
>>> print(f"pi ≈ {pi:.4f}")
pi ≈ 3.1416

>>> price = 9.5
>>> print(f"Price: ${price:.2f}")
Price: $9.50

Field width and alignment:

>>> for i in range(1, 4):
...     print(f"{i:3}  {i**2:5}")
...
  1      1
  2      4
  3      9

- ``:3`` means: right-align in a field of width 3.
- ``:<10`` means: left-align in a field of width 10.
- ``:^10`` means: center in a field of width 10.

Integer formatting:

>>> n = 255
>>> print(f"{n:d}")     # decimal
255
>>> print(f"{n:08b}")   # binary, zero-padded to 8 digits
11111111
>>> print(f"{n:x}")     # hexadecimal
ff

Older Formatting Methods

You may encounter two older formatting styles in existing code.

The .format() method:

print("The wall area is {} square feet.".format(wall_area))
print("Name: {0}, Age: {1}".format(name, age))

Percent-style formatting:

print("pi = %.4f" % 3.14159)

F-strings are preferred in new code because they are more readable and slightly faster.

The Painting Program Revisited

Here is the output section of the painting program using f-strings:

print(f"The wall area is {wall_area} square feet.")
print(f"The ceiling area is {ceiling_area} square feet.")

To display the areas with exactly one decimal place:

print(f"The wall area is {wall_area:.1f} square feet.")
print(f"The ceiling area is {ceiling_area:.1f} square feet.")