I am reading through a python script that takes an input of XML files and outputs an XML file. However, I do not understand the printing syntax. Can someone please explain what f in print(f"...") does?
args = parser.parser_args() print(f"Input directory: {args.input_directory}") print(f"Output directory: {args.output_directory}") 6 Answers
The f means Formatted string literals and it's new in Python 3.6.
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.
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 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 Even cooler is the ability to nest and format. Example date
from datetime import datetime lookup = { '01': 'st', '21': 'st', '31': 'st', '02': 'nd', '22': 'nd', '03': 'rd', '23': 'rd' } print(f"{datetime.now(): %B %d{lookup.get('%B', 'th')} %Y}") # April 14th 2022 Pretty formatting is also easier
tax = 1234 print(f'{tax:,}') # separate 1k \w comma # 1,234 print(f'{tax:,.2f}') # all two decimals # 1,234.00 print(f'{tax:~>8}') # pad left with ~ to fill eight characters or < other direction # ~~~~1234 print(f'{tax:~^20}') # centre and pad # ~~~~~~~~1234~~~~~~~~ The __format__ allows you to funk with this feature. Example
class Money: def __init__(self, currency='€'): self.currency = currency def __format__(self, value): return f"{self.currency} {float(value):.2f}" tax = 12.34 money = Money(currency='$') print(f'{money: {tax}}') # $ 12.34 There is much more. Readings:
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
1A 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) f-string in python lets you format data for printing using string templates.
Below example will help you to clarify
With f-string
name = 'Niroshan' age = 25; print(f"Hello I'm {name} and {age} years young") Hello I'm Niroshan and 25 years young
Without f-string
name = 'Niroshan' age = 25; print("Hello I'm {name} and {age} years young") Hello I'm {name} and {age} years young
