fstring

fstring

>>> name = "Eric"
>>> age = 74
>>> f"Hello, {name}. You are {age}."
'Hello, Eric. You are 74.'

>>> F"Hello, {name}. You are {age}."
'Hello, Eric. You are 74.'

>>> f"{2 * 37}"
'74'

也可以调用子程序

>>> def to_lowercase(input):
...     return input.lower()
>>> name = "Eric Idle"
>>> f"{to_lowercase(name)} is funny."
'eric idle is funny.'

>>> f"{name.lower()} is funny."
'eric idle is funny.'

也可以用在类中

class Comedian:
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name
        self.age = age

    def __str__(self):
        return f"{self.first_name} {self.last_name} is {self.age}."

    def __repr__(self):
        return f"{self.first_name} {self.last_name} is {self.age}. Surprise!"
        
>>> new_comedian = Comedian("Eric", "Idle", "74")
>>> f"{new_comedian}"
'Eric Idle is 74.'

>>> f"{new_comedian}"
'Eric Idle is 74.'
>>> f"{new_comedian!r}"
'Eric Idle is 74. Surprise!'

也可以用在多行中

>>> name = "Eric"
>>> profession = "comedian"
>>> affiliation = "Monty Python"
>>> message = (
...     f"Hi {name}. "
...     f"You are a {profession}. "
...     f"You were in {affiliation}."
... )
>>> message
'Hi Eric. You are a comedian. You were in Monty Python.'

但是前面必须要加f""，不然如下:

>>> message = (
...     f"Hi {name}. "
...     "You are a {profession}. "
...     "You were in {affiliation}."
... )
>>> message
'Hi Eric. You are a {profession}. You were in {affiliation}.'

也可以这么写：

>>> message = f"Hi {name}. " \
...           f"You are a {profession}. " \
...           f"You were in {affiliation}."
...
>>> message
'Hi Eric. You are a comedian. You were in Monty Python.'

但是这么写""" """则不行，会默认你在换行。

>>> message = f"""
...     Hi {name}. 
...     You are a {profession}. 
...     You were in {affiliation}.
... """
...
>>> message
'\n    Hi Eric.\n    You are a comedian.\n    You were in Monty Python.\n'

import datetime
 
today = datetime.datetime.today()
print(f"{today:%B %d, %Y}")
--> April 14, 2023

不能在fstring中使用''

f"newline: {ord('\n')}"

Traceback (most recent call last):
  Python Shell, prompt 29, line 1
Syntax Error: f-string expression part cannot include a backslash: , line 1, pos 0

old style 1: "Hello, %s." % name

name = "Eric"
"Hello, %s." % name
--> 'Hello, Eric.'

>>> name = "Eric"
>>> age = 74
>>> "Hello, %s. You are %s." % (name, age)
'Hello Eric. You are 74.'

如下情况会很臃肿：

>>> first_name = "Eric"
>>> last_name = "Idle"
>>> age = 74
>>> profession = "comedian"
>>> affiliation = "Monty Python"
>>> "Hello, %s %s. You are %s. You are a %s. You were a member of %s." % (first_name, last_name, age, profession, affiliation)
'Hello, Eric Idle. You are 74. You are a comedian. You were a member of Monty Python.'

old style 2: str.format()

>>> "Hello, {}. You are {}.".format(name, age)
'Hello, Eric. You are 74.'

>>> "Hello, {1}. You are {0}.".format(age, name)
'Hello, Eric. You are 74.'

>>> person = {'name': 'Eric', 'age': 74}
>>> "Hello, {name}. You are {age}.".format(name=person['name'], age=person['age'])
'Hello, Eric. You are 74.'

>>> person = {'name': 'Eric', 'age': 74}
>>> "Hello, {name}. You are {age}.".format(**person)
'Hello, Eric. You are 74.'

如下情况会很臃肿：

>>> first_name = "Eric"
>>> last_name = "Idle"
>>> age = 74
>>> profession = "comedian"
>>> affiliation = "Monty Python"
>>> print(("Hello, {first_name} {last_name}. You are {age}. " + 
>>>        "You are a {profession}. You were a member of {affiliation}.") \
>>>        .format(first_name=first_name, last_name=last_name, age=age, \
>>>                profession=profession, affiliation=affiliation))
'Hello, Eric Idle. You are 74. You are a comedian. You were a member of Monty Python.'

参考：
[1] https://realpython.com/python-f-strings/#old-school-string-formatting-in-python
[2] https://www.geeksforgeeks.org/formatted-string-literals-f-strings-python/
[3] https://stackoverflow.com/questions/51775950/why-isnt-it-possible-to-use-backslashes-inside-the-braces-of-f-strings-how-can