What's the difference between f"{x}" and "{}".format(x) in python?-CodePudding

I'm new to python, and I just want to know the difference between those examples. Does it differ when it comes to the execution speed?

When do I use one of them instead of the other?

x: int = 64
print(f"Your Number is {x}")

and

x: int = 64
txt = "Your Number is {}"
print(txt.format(x))

Thank you in advance!

CodePudding user response：

Python f-strings were added in 3.6. Therefore you should consider using format() if you need compatibility with earlier versions. Otherwise, use f-strings.

On macOS 12.1 running 3 GHz 10-Core Intel Xeon W and Python 3.10.2, f-strings are significantly faster (~60%)

CodePudding user response：

Well, personally I use f string all the time, except when I'm dealing with floats or things like that, that require a specific formatting, that's when using .format is more suitable.

But if you are not dealing with text that require a specific formatting you should use f string, its easier to read.

CodePudding user response：

There is no difference, technically speaking. The f-string format is recommended because it is more recent: it was introduced in Python 3.6. RealPython explains that f-strings are faster than str.format().

With f-strings the syntax is less verbose. Suppose you have the following variables:

first_name = "Eric"
last_name = "Idle"
age = 74
profession = "comedian"
affiliation = "Monty Python"

This is how you would format a str.format() statement.

print(("Hello, {name} {surname}. You are {age}. "   
       "You are a {profession}. You were a member of {affiliation}.") \
       .format(name=first_name, surname=last_name, age=age,\
               profession=profession, affiliation=affiliation))

With formatting strings, it is considerably shortened:

print(f"Hello {first_name} {last_name}. You are {age}"  
      f"You are a {profession}. You were a member of {affiliation}.")

Not only that: formatting strings offer a lot of nifty tricks, because they are evaluated at runtime:

>>> name="elvis" # note it is lowercase
>>> print(f"WOW THAT IS {name.upper()}")
'WOW THAT IS ELVIS'

This can be done inside a str.format(...) statement too, but f-strings make it cleaner and less cumbersome. Plus, you can also specify formatting inside the curly braces:

>>> value=123
>>> print(f"{value=}")
'value = 123'

Which normally you should have written as print("value = {number}".format(number=value)). Also, you can evaluate expressions:

>>> print(f"{value % 2 =}")
'value % 2 = 1`

And also format numbers:

>>> other_value = 123.456
>>> print(f"{other_value:.2f}") # will just print up to the second digit
'123.45'

And dates:

>>> from datetime.datetime import now
>>> print(f"{now=:%Y-%m-%d}")
'now=2022-02-02'