How to programatically generate fstring format spec?-CodePudding

How can I programmatically generate an f-string format specification? For example I'd like to zero pad this binary number up to n bits:

>> n = 5 
>> f"{6:05b}"
00110    
>> n = 7 
>> f"{6:07b}"
0000110

CodePudding user response：

Nested formatting works with f-strings as it did with the older format function, as detailed in the Format String Syntax section of the docs:

A format_spec field can also include nested replacement fields within it. These nested replacement fields may contain a field name, conversion flag and format specification, but deeper nesting is not allowed. The replacement fields within the format_spec are substituted before the format_spec string is interpreted. This allows the formatting of a value to be dynamically specified.

Sadly, the docs don't include an example with f-strings. Here's one. Notice the nested curly braces:

for n in range(3, 8):
    print(f"{6:0{n}b}")

Prints:

CodePudding user response：

You can do it by defining the f-string to be the result of a function. This will delay its evaluation until the function is called:

template = lambda: f"{6:0{n}b}"

n = 5
print(template())  # -> 00110
n = 7
print(template())  # -> 0000110