---
tags: Python
---
# Formatted string literals (f-string)

[TOC]

[PEP498 Literal String Interpolation](https://www.python.org/dev/peps/pep-0498/) f-string 是 Python3.6 引入的新方法，可以先看 [Python 3's f-Strings: An Improved String Formatting Syntax (Guide)](https://realpython.com/python-f-strings/) ，有詳盡的範例。

* 相較於 `format` 更簡潔：

    'Hello, {name}. You are {age}.'.format(name=name, age=age)

    f'Hello, {name}. You are {age}.'

* 效能也更高，比 format 快了一倍。
> F-strings provide a way to embed expressions inside string literals, using a minimal syntax. It should be noted that an f-string is really an expression evaluated at run time, not a constant value. In Python source code, an f-string is a literal string, prefixed with 'f', which contains expressions inside braces. The expressions are replaced with their values.

* pylint 需要升級到 2.x ，否則會被視為 `syntax-error` 。
expression 內變數未定義時， pylint 能夠正確抓到錯誤：`undefined-variable` 。

## datetime

>>> import datetime
>>> now = datetime.datetime.now()

>>> f'{now}'
'2019-07-18 14:44:15.942799'

>>> f'{now!r}'  # 表示 repr()，representation 會顯示更詳盡的資訊
'datetime.datetime(2019, 7, 18, 14, 44, 15, 942799)'

>>> f'{now:%r}'
'02:44:15 PM'

>>> f'{now:%X}'
'14:44:15'

>>> f'{now:%F}'
'2019-07-18'

更多格式可以看：[datetime - `strftime()` and `strptime()` Behavior](https://docs.python.org/3/library/datetime.html#strftime-and-strptime-behavior)

## 數字

>>> value = 123.456

>>> f'{value:.2f}'  # 小數點後兩位
'123.46'

>>> f'{value:8.2f}'  # 靠右對齊，寬度為 8
'  123.46'

>>> f'{value:<8.2f}'  # 靠左對齊，寬度為 8
'123.46  '

>>> f'{value:^8.2f}'  # 置中對齊，寬度為 8
' 123.46 '

>>> f'{value:09.3f}'  # 前面補零
'00123.456'

>>> f'{value:%}'  # 以百分比的方式輸出
'12345.600000%'

>>> f'{value:+}'  # 輸出包含正負號
'+123.456'

## for

>>> for x in range(4):
>>>     print(f"{'*' * x}")
*
**
***

>>> for x in range(4):
>>>     print(f"{'*' * x:>4}")  ## 靠右對齊
   *
  **
 ***
 
>>> for x in range(1, 6, 2):
>>>     print(f"{'*' * x: ^5}")  ## 置中對齊
  *
 ***
*****

## Python3.8 Debugging

>>> name = 'Celine'
>>> f'name = {name}'  # No need anymore
>>> "name = 'Celine'"

>>> f'{name = }'
"name = 'Celine'"

>>> f'{name=}'
"name='Celine'"

## 參考資料

1. [Python 3's f-Strings: An Improved String Formatting Syntax (Guide)](https://realpython.com/python-f-strings/)
2. [f-string debugging in Python 3.8](https://tirkarthi.github.io/programming/2019/05/08/f-string-debugging.html)