What is print(f"...")

Question

The f means Formatted string literals and it's new in Python 3.6.

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A formatted string literal or f-string is a string literal that is prefixed with 'f' or 'F'. These strings may contain replacement fields, which are expressions delimited by curly braces {}. While other string literals always have a constant value, formatted strings are really expressions evaluated at run time.

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Some examples of formatted string literals:

>>> name = "Fred"
>>> f"He said his name is {name}."
"He said his name is Fred."

>>> name = "Fred"
>>> f"He said his name is {name!r}."
"He said his name is Fred."

>>> f"He said his name is {repr(name)}." # repr() is equivalent to !r
"He said his name is Fred."

>>> width = 10
>>> precision = 4
>>> value = decimal.Decimal("12.34567")
>>> f"result: {value:{width}.{precision}}" # nested fields
result: 12.35

>>> today = datetime(year=2017, month=1, day=27)
>>> f"{today:%B %d, %Y}" # using date format specifier
January 27, 2017

>>> number = 1024
>>> f"{number:#0x}" # using integer format specifier
0x400

the f string is also known as the literal string to insert a variable into the string and make it part so instead of doing

x = 12
y = 10

word_string = x + ' plus ' + y + 'equals: ' + (x+y)

instead, you can do

x = 12
y = 10

word_string = f'{x} plus {y} equals: {x+y}'
output: 12 plus 10 equals: 22

this will also help with spacing due to it will do exactly as the string is written

In Python 3.6, the f-string was introduced(PEP 498). In short, it is a way to format your string that is more readable and fast.

Example:

agent_name = 'James Bond'
kill_count = 9

# old ways
print('{0} has killed {1} enemies '.format(agent_name,kill_count))

# f-strings way
print(f'{agent_name} has killed {kill_count} enemies')

The f or F in front of strings tell Python to look at the values inside {} and substitute them with the variables values if exists. The best thing about f-formatting is that you can do cool stuff in {}, e.g. {kill_count * 100}.

You can use it to debug using print e.g.

print(f'the {agent_name=}.')
# the agent_name='James Bond'

Formatting, such as zero-padding, float and percentage rounding is made easier:

print(f'{agent_name} shoot with {9/11 : .2f} or {9/11: .1%} accuracy')
# James Bond shoot with  0.82 or  81.8% accuracy 

There is much more. Readings:

• PEP 498 Literal String Interpolation
• Python String Formatting

A string prefixed with 'f' or 'F' and writing expressions as {expression} is a way to format string, which can include the value of Python expressions inside it.

Take these code as an example:

def area(length, width):
    return length * width

l = 4
w = 5

print("length =", l, "width =", w, "area =", area(l, w))  # normal way
print(f"length = {l} width = {w} area = {area(l,w)}")     # Same output as above
print("length = {l} width = {w} area = {area(l,w)}")      # without f prefixed

Output:

length = 4 width = 5 area = 20
length = 4 width = 5 area = 20
length = {l} width = {w} area = {area(l,w)}

args = parser.parser_args()

print(f"Input directory: {args.input_directory}")
print(f"Output directory: {args.output_directory}")

is the same as

print("Input directory: {}".format(args.input_directory))
print("Output directory: {}".format(args.output_directory))

it is also the same as

print("Input directory: "+args.input_directory)
print("Output directory: "+args.output_directory)