如何为新的 python 格式选项设置可变精度

问题描述

我喜欢包含变量的字符串的新格式选项，但我希望有一个变量来设置整个脚本的精度，但我不知道该怎么做.举个小例子:

I am loving the new format option for strings containing variables, but I would like to have a variable that sets the precision through out my script and I am not sure how to do that. Let me give a small example:

a = 1.23456789
out_str = 'a = {0:.3f}'.format(a)
print(out_str)

现在这就是我想用伪代码做的事情:

Now this is what I would want to do in pseudo code:

a = 1.23456789
some_precision = 5
out_str = 'a = {0:.(some_precision)f}'.format(a)
print(out_str)

但我不确定，是否可能以及语法是否可能.

but I am not sure, if it is possibly and if it is possibly how the syntax would look like.

推荐答案

您可以嵌套占位符，其中嵌套的占位符可以在格式规范中的任何位置使用:

You can nest placeholders, where the nested placeholders can be used anywhere in the format specification:

out_str = 'a = {0:.{some_precision}f}'.format(a, some_precision=some_precision)

我在那里使用了命名占位符，但您也可以使用编号插槽:

I used a named placeholder there, but you could use numbered slots too:

out_str = 'a = {0:.{1}f}'.format(a, some_precision)

也支持嵌套槽(Python 2.7 及更高版本)的自动编号；编号仍然从左到右进行:

Autonumbering for nested slots (Python 2.7 and up) is supported too; numbering still takes place from left to right:

out_str = 'a = {0:.{}f}'.format(a, some_precision)

嵌套槽先被填充；当前的实现允许您最多嵌套 2 层占位符，因此在占位符中使用占位符是行不通的:

Nested slots are filled first; the current implementation allows you to nest placeholders up to 2 levels, so using placeholders in placeholders in placeholders doesn't work:

>>> '{:.{}f}'.format(1.234, 2)
'1.23'
>>> '{:.{:{}}f}'.format(1.234, 2, 'd')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Max string recursion exceeded

您也不能在字段名称中使用占位符(因此不能将值动态分配给插槽).

You also can't use placeholders in the field name (so no dynamic allocation of values to slots).

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