如何在 Python 格式说明符中使用变量名

Question

Is it possible to use a variable inside of a Python string formatting specifier?

I have tried:

display_width = 50
print('\n{:^display_width}'.format('some text here'))

but get a ValueError: Invalid format specifier . I have also tried display_width = str(50)

however, just entering print('\\n{:^50}'.format('some text here')) works just fine.

Answer 1

Yes, but you have to pass them in as arguments to format , and then refer to them wrapped in {} like you would the argument name itself:

print('\n{:^{display_width}}'.format('some text here', display_width=display_width))

Or shorter but a little less explicit:

print('\n{:^{}}'.format('some text here', display_width))

Since this question was originally posted, Python 3.6 has added f-strings, which allow you to do this without using the format method and it uses variables which are in scope rather than having to pass in the named variables as keyword arguments:

display_width = 50
text = 'some text here'
print(f'\n{text:^{display_width}}')

Answer 2

Python f-string is more flexible.

>>> display_width = 50
>>> display_content = "some text here"
>>> print(f'\n{display_content:^{display_width}}')

                  some text here

Answer 3

It seems like since this question was first posted, python introduced f-strings. See this web page for info.

>>> name = 'Fred'
>>> age = 42
>>> f'He said his name is {name} and he is {age} years old.'
He said his name is Fred and he is 42 years old.

Answer 4

也许

print(('{0:^'+str(display_width)+'}').format('hello'))