8.11: f-string formatting

f-strings are the most recent and easiest way to format output, and so we will cover them first.  You simply place an f in front of the quotes, and use curly brackets to insert variable names.

• Introduced in Python 3.6
• Place f in front of string
• allows you to skip the .format() step

 Analyze the code, then run it, and then answer the questions below it.

molecule ="hydrogen"
molarmass=1.00794
mass = 10
moles =mass/molarmass
print(f'{mass} grams of the molecule {molecule} has {moles} moles.\n \t{mass} \
grams of the molecule {molecule} has {moles} moles. \n \
\t\t{mass} grams of the molecule {molecule} has {moles} moles.\n \
\t\t\t the molar mass of hydrogen is {molarmass}.\n \
This is the last line.')

Hello world!

 

Exercise \(\PageIndex{15.a}\)

Answer the following questions concerning the above code

1. Which variable is a string?
2. Which variable is an integer?
3. Which variables are floats?
4. What does \ do?
5. What does \n do?
6. What does \t do?

Answer a

molecule

Answer b

mass

Answer c

molar mass and moles, which is an integer divided by a float

Answer d

Allows you to extend a long line of python script to the next line (this f-string is very long).  The best practice is not to have a line of code longer than 80 characters.

Answer e

Next line (like the carriage return on a typewritter)

Answer f

tab

 

Note that we often want to limit the number of digits displayed to the right of the decimal to represent the precision with which a number is known. In the following example we will try and show how this is done for f-strings.  we first print 20 decimal digits, so you can see what position a number is printed, and then we print the variable twice, using different format conventions

variable=12345.6789
print(f'12345678901234567890')
print(f'{variable:.4f}\n{variable:.2f}')

print(f'12345678901234567890')
print(f'{variable:10.2f}\n{variable:11.2f} \n{variable:11.3f}')

Hello world!

 

Exercise \(\PageIndex{1}\)

consider the expression  {variable_name:xx.yyf}

1. What does the colon indiate?
2. What does the xx indicate?
3. What does the yy indicate?
4. What does the f indicate?

Answer a

The colon indicates the following statement describes the formatting of the variable in the print statement

Answer b

This tells you the width of the number.  This allows you to right justify all numbers.

Answer c

This tells you the precision of the number, that is, how many values to the right of the decimal are expressed in the print statement

Answer d

this is a floating decimal number

 

Exercise \(\PageIndex{15.a}\)

Consider the following code and answer question (a) before running it

print(f"How long is this number going to be {1234.5678:2.3f}")

Hello world!

 

1. The :2.3f has a width of 2 and a precision of 3, so how long will the number be printed out?
2. Which variable is an integer?
3. Which variables are floats?
4. What does \ do?
5. What does \n do?
6. What does \t do?

Answer a

The entire number will be printed to the third decimal, so 8 characters including the dot.

Answer b

mass

Answer c

molar mass and moles, which is an integer divided by a float

Answer d

Allows you to extend a long line of python script to the next line (this f-string is very long).  The best practice is not to have a line of code longer than 80 characters.

Answer e

Next line (like the carriage return on a typewritter)

Answer f

tab