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Working with Python

String Formatting Cookbook

Python v2.7 introduced a new string formatting method, that is now the default in Python3. I started this string formatting cookbook as a quick reference to help me format numbers and strings. Thanks to other contributors I've expanded the examples over time.

Python 3.6 introduced, formatted string literals, often referred to as f-strings as another method to help format strings. It is simpler to prepend an f to the string then append .format(). Using f-strings in Python is similar to JavaScript's template literals, if you are familiar with them.

Here's an example comparing the three ways to format a float number:

pi = 3.14159
print(" pi: %1.2f " % pi)         # older
print(" pi: {:.2f}".format( pi )) # .format()
print(f" pi: {pi:.2f}")           # f-string

Number formatting

This table shows various ways to format numbers using Python's str.format() and formatted string literals, including examples for both float formatting and integer formatting.

To run examples use: print(f"{NUM:FORMAT}") or print("{:FORMAT}".format(NUM));

NumberFormatOutputDescription
3.1415926{:.2f}3.14Format float 2 decimal places
3.1415926{:+.2f}+3.14Format float 2 decimal places with sign
-1{:+.2f}-1.00Format float 2 decimal places with sign
2.71828{:.0f}3Format float with no decimal places
5{:0>2d}05Pad number with zeros (left padding, width 2)
5{:x<4d}5xxxPad number with x's (right padding, width 4)
1000000{:,}1,000,000Number format with comma separator
0.25{:.2%}25.00%Format percentage
1000000000{:.2e}1.00e+09Exponent notation
13{:10d}        13Right aligned (default, width 10)
13{:<10d}13Left aligned (width 10)
13{:^10d}    13Center aligned (width 10)

String .format() basics

Here are a couple of examples of basic string substitution, the {} is the placeholder for substituted variables. If no format is specified, it will insert and format as a string.

s1 = "show me the {}".format("money")
s2 = "hmmm, this is a {} {}".format("tasty", "burger")

With formatted string literals, this is simply:

s1 = f"show me the {money}"
s2 = f"hmmm, this is a {tasty} {burger}"

Substitution positioning

A benefit of .format() not available in f-strings is using the numeric position of the variables and changing them in the strings, this gives flexibility, if you make a mistake in the order you can easily correct without shuffling all the variables around.

s1 = " {0} is better than {1} ".format("emacs", "vim")
s2 = " {1} is better than {0} ".format("emacs", "vim")

Variable formatting

Use {} as a variable inside the formatting brackets (h/t Peter Beens for tip). This example uses a precision variable to control how many decimal places to show:

pi = 3.1415926
precision = 4
print( "{:.{}f}".format( pi, precision ) )
>>> 3.1415

Older % string formatter

An example comparing variable substitution with the older % method vs. .format():

s1 = "cats"
s2 = "dogs"
s3 = " %s and %s living together" % (s1, s2)
s4 = " {} and {} living together ".format(s1, s2)

Using the older format method, I would often get the errors:

TypeError: not enough arguments for format string

or

TypeError: not all arguments converted during string formatting

because I miscounted my substitution variables, doing something like the following made it easy to miss a variable.

Using one of the new Python string formatters you can use numbered parameters so you don't have to count how many you have, at least on half of it.

set = " ({0}, {1}, {2}, {3}, {4}, {5}, {6}, {7}) ".format(a,b,c,d,e,f,g)

Formatted string literals

As shown above, formatted string literals, or f-strings, use a shorter syntax making it easier and more template-like. F-strings also support functions inside of the brackets { } this allows you to:

Do math with f-strings:

print( f"Do math: 3 * 6 = {3 * 6}" )
>>> Do math: 3 * 6: =18

Call functions with f-strings;

verb = "runs"
print( f"The girl {verb.upper()} quickly." )
>>> The girl RUNS quickly.

Delimiting f-strings

Use f-strings with the three different type of quotation marks in Python, single, double, or triple quotes. The following will all output the same:

name = "Fred"
print( f'{name}' )
print( f"{name}" )
print( f"""{name}""" )

F-String error

The one thing to be careful with is mixing the two formats, if {} are used inside of an f-string, it will give the error:

SyntaxError: f-string: empty expression not allowed

Each set of brackets used in an f-string requires a value or variable.

Formatting tips with .format()

The format() function offers additional features and capabilities, here are a few useful tips and tricks to format strings in Python:

Reuse same variable multiple times

Using % to format requires a strict ordering of variables, the .format() method allows you to put them in any order as well as repeating for reuse.

"Oh {0}, {0}! wherefore art thou {0}?".format("Romeo")
>>> 'Oh Romeo, Romeo! wherefore art thou Romeo?'

Convert values to different bases

A surprising use with the string format command is to convert numbers to different bases. Use the letter in the formatter to indicate the number base: decimal, hex, octal, or binary.

This example formats the number 21 in each base:

"{0:d} - {0:x} - {0:o} - {0:b} ".format(21)
>>> 21 - 15 - 25 - 10101

Use format as a function

Use .format as a function to separate text and formatting from code. For example, at the beginning of a program include all the formats for later use.

## defining formats
email_f = "Your email address was {email}".format
 
## use elsewhere
print(email_f(email="bob@example.com"))

Hat tip to earthboundkids who provided this on reddit.

Using format as a function can be used to adjust formating by user preference.

## set user preferred format
num_format = "{:,}".format
 
## use elsewhere
print(num_format(1000000))

Internationalization

To use locale specific formatting for numbers, first set the locale, and then use the formating code n instead of d. For example, using commas or periods to separate thousands in numbers based on the user's locale.

Here is an example, setting locale and formatting a number to display the proper separator:

import locale
locale.setlocale(locale.LC_ALL, '')
 
print("{:n}".format(1000000))

Escaping braces

If you need to use braces when using str.format() just double them up:

print(" The {} set is often represented as {% raw %}{{0}}{% endraw %}".format("empty"))
~~ The empty set is often represented as {0}

Table formatting data

Use the width and the left and right justification to align your data into a nice table format. Here's an example to show how to format:

# data
starters = [
    [ 'Andre Iguodala', 4, 3, 7 ],
    [ 'Klay Thompson', 5, 0, 21 ],
    [ 'Stephen Curry', 5, 8, 36 ],
    [ 'Draymon Green', 9, 4, 11 ],
    [ 'Andrew Bogut', 3, 0, 2 ],
]
 
# define format row
row = "| {player:<16s} | {reb:2d} | {ast:2d} | {pts:2d} |".format
 
for p in starters:
    print(row(player=p[0], reb=p[1], ast=p[2], pts=p[3]))

This would output:

 | Andre Iguodala   |  4 |  3 |  7 |
 | Klay Thompson    |  5 |  0 | 21 |
 | Stephen Curry    |  5 |  8 | 36 |
 | Draymon Green    |  9 |  4 | 11 |
 | Andrew Bogut     |  3 |  0 |  2 |

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