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}")
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
forF. 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=2023, month=1, day=27)
>>> f"{today:%B %d, %Y}" # using date format specifier
January 27, 2023
>>> number = 1024
>>> f"{number:#0x}" # using integer format specifier
0x400
In Python 3.6, the f-string, formatted string literal, 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("%s has killed %d enemies" % (agent_name,kill_count))
print('{} has killed {} enemies'.format(agent_name,kill_count))
print('{name} has killed {kill} enemies'.format(name=agent_name,kill=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 , expressions or instance inside {} and substitute them with the variables values or results 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
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)
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
f function is Transferring data to other places in your content. It mostly used Changeable Data.
class Methods (): def init(self ,F,L,E,S,T): self.FirstName=F self.LastName=L self.Email=E self.Salary=S self.Time =T
def Salary_Msg(self):
#f is called function f
#next use {write in}
return f{self.firstname} {self.Lastname} earns AUD {self.Salary}per {self.Time} "
