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2.3 Formatting
This section is a slight digression, but when you work with data, you often want to produce structured output (tables, etc.). For example:
Name Shares Price
---------- ---------- -----------
AA 100 32.20
IBM 50 91.10
CAT 150 83.44
MSFT 200 51.23
GE 95 40.37
MSFT 50 65.10
IBM 100 70.44
String Formatting
One way to format string in Python 3.6+ is with f-strings.
>>> name = 'IBM'
>>> shares = 100
>>> price = 91.1
>>> f'{name:>10s} {shares:>10d} {price:>10.2f}'
' IBM 100 91.10'
>>>
The part {expression:format} is replaced.
It is commonly used with print.
print(f'{name:>10s} {shares:>10d} {price:>10.2f}')
Format codes
Format codes (after the : inside the {}) are similar to C printf(). Common codes
include:
d Decimal integer
b Binary integer
x Hexadecimal integer
f Float as [-]m.dddddd
e Float as [-]m.dddddde+-xx
g Float, but selective use of E notation
s String
c Character (from integer)
Common modifiers adjust the field width and decimal precision. This is a partial list:
:>10d Integer right aligned in 10-character field
:<10d Integer left aligned in 10-character field
:^10d Integer centered in 10-character field
:0.2f Float with 2 digit precision
Dictionary Formatting
You can use the format_map() method to apply string formatting to a dictionary of values:
>>> s = {
'name': 'IBM',
'shares': 100,
'price': 91.1
}
>>> '{name:>10s} {shares:10d} {price:10.2f}'.format_map(s)
' IBM 100 91.10'
>>>
It uses the same codes as f-strings but takes the values from the
supplied dictionary.
format() method
There is a method format() that can apply formatting to arguments or
keyword arguments.
>>> '{name:>10s} {shares:10d} {price:10.2f}'.format(name='IBM', shares=100, price=91.1)
' IBM 100 91.10'
>>> '{:>10s} {:10d} {:10.2f}'.format('IBM', 100, 91.1)
' IBM 100 91.10'
>>>
Frankly, format() is a bit verbose. I prefer f-strings.
C-Style Formatting
You can also use the formatting operator %.
>>> 'The value is %d' % 3
'The value is 3'
>>> '%5d %-5d %10d' % (3,4,5)
' 3 4 5'
>>> '%0.2f' % (3.1415926,)
'3.14'
This requires a single item or a tuple on the right. Format codes are
modeled after the C printf() as well.
Note: This is the only formatting available on byte strings.
>>> b'%s has %d messages' % (b'Dave', 37)
b'Dave has 37 messages'
>>> b'%b has %d messages' % (b'Dave', 37) # %b may be used instead of %s
b'Dave has 37 messages'
>>>
Exercises
Exercise 2.8: How to format numbers
A common problem with printing numbers is specifying the number of decimal places. One way to fix this is to use f-strings. Try these examples:
>>> value = 42863.1
>>> print(value)
42863.1
>>> print(f'{value:0.4f}')
42863.1000
>>> print(f'{value:>16.2f}')
42863.10
>>> print(f'{value:<16.2f}')
42863.10
>>> print(f'{value:*>16,.2f}')
*******42,863.10
>>>
Full documentation on the formatting codes used f-strings can be found
here. Formatting
is also sometimes performed using the % operator of strings.
>>> print('%0.4f' % value)
42863.1000
>>> print('%16.2f' % value)
42863.10
>>>
Documentation on various codes used with % can be found
here.
Although it’s commonly used with print, string formatting is not tied to printing.
If you want to save a formatted string. Just assign it to a variable.
>>> f = '%0.4f' % value
>>> f
'42863.1000'
>>>
Exercise 2.9: Collecting Data
In Exercise 2.7, you wrote a program called report.py that computed the gain/loss of a
stock portfolio. In this exercise, you’re going to start modifying it to produce a table like this:
Name Shares Price Change
---------- ---------- ---------- ----------
AA 100 9.22 -22.98
IBM 50 106.28 15.18
CAT 150 35.46 -47.98
MSFT 200 20.89 -30.34
GE 95 13.48 -26.89
MSFT 50 20.89 -44.21
IBM 100 106.28 35.84
In this report, “Price” is the current share price of the stock and “Change” is the change in the share price from the initial purchase price.
In order to generate the above report, you’ll first want to collect
all of the data shown in the table. Write a function make_report()
that takes a list of stocks and dictionary of prices as input and
returns a list of tuples containing the rows of the above table.
Add this function to your report.py file. Here’s how it should work
if you try it interactively:
>>> portfolio = read_portfolio('Data/portfolio.csv')
>>> prices = read_prices('Data/prices.csv')
>>> report = make_report(portfolio, prices)
>>> for r in report:
print(r)
('AA', 100, 9.22, -22.980000000000004)
('IBM', 50, 106.28, 15.180000000000007)
('CAT', 150, 35.46, -47.98)
('MSFT', 200, 20.89, -30.339999999999996)
('GE', 95, 13.48, -26.889999999999997)
...
>>>
Exercise 2.10: Printing a formatted table
Redo the for-loop in Exercise 2.9, but change the print statement to format the tuples.
>>> for r in report:
print('%10s %10d %10.2f %10.2f' % r)
AA 100 9.22 -22.98
IBM 50 106.28 15.18
CAT 150 35.46 -47.98
MSFT 200 20.89 -30.34
...
>>>
You can also expand the values and use f-strings. For example:
>>> for name, shares, price, change in report:
print(f'{name:>10s} {shares:>10d} {price:>10.2f} {change:>10.2f}')
AA 100 9.22 -22.98
IBM 50 106.28 15.18
CAT 150 35.46 -47.98
MSFT 200 20.89 -30.34
...
>>>
Take the above statements and add them to your report.py program.
Have your program take the output of the make_report() function and print a nicely formatted table as shown.
Exercise 2.11: Adding some headers
Suppose you had a tuple of header names like this:
headers = ('Name', 'Shares', 'Price', 'Change')
Add code to your program that takes the above tuple of headers and creates a string where each header name is right-aligned in a 10-character wide field and each field is separated by a single space.
' Name Shares Price Change'
Write code that takes the headers and creates the separator string between the headers and data to follow. This string is just a bunch of “-“ characters under each field name. For example:
'---------- ---------- ---------- -----------'
When you’re done, your program should produce the table shown at the top of this exercise.
Name Shares Price Change
---------- ---------- ---------- ----------
AA 100 9.22 -22.98
IBM 50 106.28 15.18
CAT 150 35.46 -47.98
MSFT 200 20.89 -30.34
GE 95 13.48 -26.89
MSFT 50 20.89 -44.21
IBM 100 106.28 35.84
Exercise 2.12: Formatting Challenge
How would you modify your code so that the price includes the currency symbol ($) and the output looks like this:
Name Shares Price Change
---------- ---------- ---------- ----------
AA 100 $9.22 -22.98
IBM 50 $106.28 15.18
CAT 150 $35.46 -47.98
MSFT 200 $20.89 -30.34
GE 95 $13.48 -26.89
MSFT 50 $20.89 -44.21
IBM 100 $106.28 35.84