如何在 Python 格式说明符中使用变量名

问题描述

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

I have tried:

display_width = 50
print('
{:^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(' {:^50}'.format('some text here')) works just fine.

解决方案

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

Or shorter but a little less explicit:

print('
{:^{}}'.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'
{text:^{display_width}}')