Contents

3.10. String Formatting (Interactive)

This is an interactive tutorial. You can click the rocket -> binder link at the top of this page to interact with the code and outputs below.

3.10.1. Why Format

Often string formatting is used to truncate part of a string or round floating point numbers to a desired precision. All of the techniques below use a special shorthand for specifying the formatting desired.

For example:

To round pi to 4 decimal points, we would use the float formatter, f and specify .4 decimal places. This will round to the number of decimal places specified.

pi = 3.14159265
print("Pi is %.4f" % pi)

Pi is 3.1416

To specify a certain number of characters width, you can also put a number before the decimal point in the format code.

For example:

To print the following lines as 10 characters each, we would specify %10.f as the format code and Python will automatically add spaced in front to make the output 10 characters long:

print("%10.f" % 1)
print("%10.f" % 10)
print("%10.f" % 100)
print("%10.f" % 1000)
print("%10.f" % 10000)

         1
        10
       100
      1000
     10000

3.10.2. Three types of string formatting

Python allows you to specify how to format strings in 3 ways:

  • The Old school (% operator): " "%()

  • The .format() method: " ".format()

  • With an f-string (Python version > 3.6 only): f" "

Useful sites:

  • Python string docs link

  • Using % and .format(): link

  • Python f-strings link

3.10.3. Old school (%)

Below we use the % operator after the string to include the three elements into the three locations denoted by % within the string.

We use three different format codes for the three numbers included in the string:

  • The integer format code d

  • The float format code rounding to a whole number .0f

  • The float format code rounding to 2 decimal places .2f.

Note: To print a regular %, you need to type two %% in a row when using the % operator.

discount = 30  # %
price = 499.99
new_price = price * (1-discount/100)

print("SALE: Get %d%% off the new PS5 and pay $%.0f instead of $%.2f." % 
      (discount, new_price, price))

SALE: Get 30% off the new PS5 and pay $350 instead of $499.99.

3.10.4. Format method (.format())

The format method inserts each element in the parentheses of .format(), into the brackets in the string. The same format codes used above are included after the colon in each set of brackets "{:fmt}".

print("SALE: Get {:d}% off the new PS5 and pay ${:.0f} instead of ${:.2f}."
      .format(discount, new_price, price))

SALE: Get 30% off the new PS5 and pay $350 instead of $499.99.

3.10.5. F-string (f’’)

The f-string allows us to include the element in place in the string instead of in a list at the end. This can make it easier to read which values are being inserted into which parts of the string. The syntax is {value:fmt} with the value being specified before the colon and the format code after the colon in the {} brackets.

print(f"SALE: Get {discount:d}% off the new PS5 and pay ${new_price:.0f} instead of ${price:.2f}.")

SALE: Get 30% off the new PS5 and pay $350 instead of $499.99.

3.10.6. String formatting with loop

We can loop through the following two lists of values and print each pair with a format string.

month   = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun']
balance = [-10.45,  50.99,  0.99,  -5.76,  100.57, -78.22]

print('Monthly Balance')
for mo, bal in zip(month, balance):
    print(f'We have ${bal:.2f} left in {mo}.')

Monthly Balance
We have $-10.45 left in Jan.
We have $50.99 left in Feb.
We have $0.99 left in Mar.
We have $-5.76 left in Apr.
We have $100.57 left in May.
We have $-78.22 left in Jun.

This looks a little messy. We can clean up our output by:

  • Specifying the width of the number field. Since the longest output is 6 characters, we can use the code 6.2f to have Python pad all of the numbers to 6 characters long

  • We can also make a 30-character underline with the -^30 short hand.

# Monthly Balance is centered on a 30 char line of ---
print('{:-^30}'.format('Monthly Balance'))

for mo, bal in zip(month, balance):
    print(f'We have $ {bal:6.2f} left in {mo}.')

# Here we can make a 30 char line of ===
print(f'{"":=^30}')
print('Total balance for the first {} months was $ {:.2f}'.format(len(month), sum(balance)))

-------Monthly Balance--------
We have $ -10.45 left in Jan.
We have $  50.99 left in Feb.
We have $   0.99 left in Mar.
We have $  -5.76 left in Apr.
We have $ 100.57 left in May.
We have $ -78.22 left in Jun.
==============================
Total balance for the first 6 months was $ 58.12

3.10.7. Practice: multiplication table

Try to exactly reproduce the table below by using a loop, print statements and string format codes.

Note: Pay attention to spacing, zeros in front of the single digit numbers, and the horizontal and vertical lines

01  |02  03  04  05  06  07  08  09  
------------------------------------
02  |04  06  08  10  12  14  16  18  
03  |06  09  12  15  18  21  24  27  
04  |08  12  16  20  24  28  32  36  
05  |10  15  20  25  30  35  40  45  
06  |12  18  24  30  36  42  48  54  
07  |14  21  28  35  42  49  56  63  
08  |16  24  32  40  48  56  64  72  
09  |18  27  36  45  54  63  72  81

Click the button to reveal ONE of the answers!

import numpy as np
for i in range(1,10):
    j = np.arange(1,10)

    for idx, ans in enumerate(i*j):
        if idx!=8:
            print(f'{ans:02d}  ', end='')
        else:
            print(f'{ans:02d}  ')
    
        if idx==0:
            print('|', end='')
            
    if i == 1:
        print('{}'.format('-'*(4*9)) )



# Create your multiplication table here!