print()

The function print() outputs character strings, whereby other Python data types can easily be converted into strings and formatted, for example:

>>> import math
>>> pi = math.pi
>>> d = 28
>>> u = pi * d
>>> print(
...     "Pi is",
...     pi,
...     "and the circumference with a diameter of",
...     d,
...     "inches is",
...     u,
...     "inches.",
... )
Pi is 3.141592653589793 and the circumference with a diameter of 28 inches is 87.96459430051421 inches.

F-Strings

F-strings can be used to shorten numbers that are too detailed for a text:

>>> print(f"The value of Pi is {pi:.3f}.")
The value of Pi is 3.142.

In {pi:.3f}, the format specification f is used to truncate the number Pi to three decimal places.

In A/B test scenarios, you often want to display the percentage change in a key figure. F strings can be used to formulate them in an understandable way:

>>> metrics = 0.814172
>>> print(f"The AUC has increased to {metrics:=+7.2%}")
The AUC has increased to +81.42%

In this example, the variable metrics is formatted with = taking over the contents of the variable after the +, displaying a total of seven characters including the plus or minus sign, metrics and the percent sign. .2 provides two decimal places, while the % symbol converts the decimal value into a percentage. For example, 0.514172 is converted to +51.42%.

Values can also be converted into binary and hexadecimal values:

>>> block_size = 192
>>> print(f"Binary block size: {block_size:b}")
Binary block size: 11000000
>>> print(f"Hex block size: {block_size:x}")
Hex block size: c0

There are also formatting specifications that are ideally suited for CLI output, for example:

>>> data_types = [(7, "Data types", 19), (7.1, "Numbers", 19), (7.2, "Lists", 23)]
>>> for n, title, page in data_types:
...     print(f"{n:.1f} {title:.<25} {page: >3}")
...
7.0 Data types...............  19
7.1 Numbers..................  19
7.2 Lists....................  23

In general, the format is as follows, whereby all information in square brackets is optional:

:[[FILL]ALIGN][SIGN][0b|0o|0x|d|n][0][WIDTH][GROUPING]["." PRECISION][TYPE]

The following table lists the fields for character string formatting and their meaning:

Field

Meaning

FILL

Character used to fill in ALIGN. The default value is a space.

ALIGN

Text alignment and fill character:

<: left-aligned
>: right-aligned
^: centred
=: Fill character after SIGN

SIGN

Display sign:

+: Display sign for positive and negative numbers
-: Default value, - only for negative numbers or space for positive

0b|0o|0x|d|n

Sign for integers:

0b: Binary numbers
0o: Octal numbers
0x: Hexadecimal numbers
d: Default value, decimal integer with base 10
n: uses the current locale setting to insert the corresponding number separators

0

fills with zeros

WIDTH

Minimum field width

GROUPING

Number separator: [1]

,: comma as thousands separator
_: underscore for thousands separator

.PRECISION
For floating point numbers, the number of digits after the point
For non-numeric values, the maximum length

TYPE

Output format as number type or string

… for integers:

b: binary format
c: converts the integer to the corresponding Unicode character
d: default value, decimal character
n: same as d, th the difference that it uses the current locale setting to insert the corresponding number separators
o: octal format
x: Hexadecimal format in base 16, using lowercase letters for the digits above 9
X: Hexadecimal format based on 16, using capital letters for digits above 9

… for floating point numbers:

e: Exponent with e as separator between coefficient and exponent
E: Exponent with E as separator between coefficient and exponent
g: Standard value for floating point numbers, whereby the exponent has a fixed width for large and small numbers
G: Like g, but changes to E if the number becomes too large. The representations of infinity and NaN are also written in capital letters
n: Like g with the difference that it uses the current locale setting to insert the corresponding number separators
%: Percentage. Multiplies the number by 100 and displays it in the fixed format f followed by a percent sign

Tip

A good source for F-strings is the help function:

>>> help()
help> FORMATTING
...

You can browse through the help here and find many examples.

You can exit the help function again with :–q and ⏎.

See also

Debugging F-Strings

In Python 3.8, a specifier was introduced to help with debugging F-string variables. By adding an equals sign =, the code is included within the F-string:

>>> uid = "veit"
>>> print(f"My name is {uid.capitalize()=}")
My name is uid.capitalize()='Veit'

Formatting date and time formats and IP addresses

datetime supports the formatting of strings using the same syntax as the strftime method for these objects.

>>> import datetime
>>> today = datetime.date.today()
>>> print(f"Today is {today:%d %B %Y}.")
Today is 26 November 2023.

The ipaddress module of Python also supports the formatting of IPv4Address and IPv6Address objects.

Finally, third-party libraries can also add their own support for formatting strings by adding a __format__ method to their objects.

See also