Formatting our output with Python’s F strings

Using F string to format the output from Python

I have recently been on a tear of different challenges on the site HackerRank. I am about halfway through their 30 days of code and 10 days of statistics. These challenges often require to output number to a certain a number of significant digits. I’ve always thought that the round function can be used for this, but I am wrong. The F string seems to be a powerful tool to accomplish this, and worth your time learning if you are unfamiliar.

Structure of an F string

The formatting of an F string starts with a f prior to quotations, whether they be single or double quotes. Any variable can then be included within a series of {}. This formatting can make it easier than turning values into strings and concatenating all strings into a single line of text. This is easily demonstrated with a large mix of values and strings.

x = 1/3
y = 1/6

print("The value is " + str(x) + " is greater than " + str(y))
print(f"The value is {x} is greater than {y}")

The value is 0.3333333333333333 is greater than 0.16666666666666666
The value is 0.3333333333333333 is greater than 0.16666666666666666

The values can then be formatted with : after the variable name. The number of digits prior and post the decimal can then be specified. The f is added after the decimal formatting to ensure the value is treated as a float.

print(f"The value is {x:.3f} is greater than {y:.2f}")

The value is 0.333 is greater than 0.17

The values passed are not specific to the number of digits, but the minimum number of spaces. This means you can ensure specific space aligned, such as for a table, by including these values.

z = [10000, 500, 10, 0.001, .1]
for i in z:
        print(f"the value is: {i:5}")

the value is: 10000
the value is:   500
the value is:    10
the value is: 0.001
the value is:   0.1

Additionally, we can add leading zeros by adding zero prior to the number of digits.

for i in z:
        print(f"the value is: {i:05}")

the value is: 10000
the value is: 00500
the value is: 00010
the value is: 0.001
the value is: 000.1

Alternative formatting

There are a few alternative methods for f strings. From my understanding, they are not as fast when it comes to performance. I don’t think that is of particular importance. If your script needs a high level of performance, than you probably don’t want many print statements.

Format Method()

The format method is very similar to f strings with the use of the {}. The string is not preceded by f and the {} can remain empty or contain position indexing. The values are then added in the .format function after the string. The order of the variable in the string will correspond with the number used in the {}, if used at all.

print("The value is {} is greater than {}".format(1/3, 1/6))

The value is 0.3333333333333333 is greater than 0.16666666666666666

Old % Method

The Old % operator (modulo) replaces the value in the string. Formatting details, such as those previously discussed, are entered after the %. The variables or values are then entered after the string when preceded by another %. Multiple values can be passed.

print("The value is %5.3f is greater than %5.3f" %(x,y))

The value is 0.333 is greater than 0.167

Conclusions

Whichever method you decide, it probably won’t make a huge difference. The important part is to understand is the actual formatting. F strings also seem to make it easier to understand the code, as the actual values are inline with the string and the formatting.

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