Insert newline after equals sign in self documenting f-string in python3.8

Question

With python3.8, a new feature is self documenting format strings. Where one would normally do this:

>>> x = 10.583005244
>>> print(f"x={x}")
x=10.583005244
>>>

One can now do this, with less repetition:

>>> x = 10.583005244
>>> print(f"{x=}")
x=10.583005244
>>>

This works very well for one line string representations. But consider the following scenario:

>>> import numpy as np
>>> some_fairly_long_named_arr = np.random.rand(4,2)
>>> print(f"{some_fairly_long_named_arr=}")
some_fairly_long_named_arr=array([[0.05281443, 0.06559171],
       [0.13017109, 0.69505908],
       [0.60807431, 0.58159127],
       [0.92113252, 0.4950851 ]])
>>>

Here, the first line does not get aligned, which is (arguably) not desirable. I would rather prefer the output of the following:

>>> import numpy as np
>>> some_fairly_long_named_arr = np.random.rand(4,2)
>>> print(f"some_fairly_long_named_arr=n{some_fairly_long_named_arr!r}")
some_fairly_long_named_arr=
array([[0.06278696, 0.04521056],
       [0.33805303, 0.17155518],
       [0.9228059 , 0.58935207],
       [0.80180669, 0.54939958]])
>>>

Here, the first line of the output is aligned as well, but it defeats the purpose of not repeating the variable name twice in the print statement.

The example is a numpy array, but it could have been a pandas dataframe etc. as well.

Hence, my question is: Can a newline character be inserted after the = sign in self documenting strings?

I tried to add it like this, but it does not work:

>>> print(f"{some_fairly_long_named_arr=n}")
SyntaxError: f-string expression part cannot include a backslash

I read the docs on format-specification-mini-language, but most of the formatting there only works for simple data types like integers, and I was not able to achieve what I wanted using those that work.

Sorry for the long write-up.

Asked By: Enigma Machine

||

Source

Answer 1

Wouldn’t recommend this at all, but for possibility’s sake:

import numpy as np

_old_array2string = np.core.arrayprint._array2string

def _array2_nice_string(*args, **kwargs):
    non_nice_string = _old_array2string(*args, **kwargs)

    dimension_strings = non_nice_string.split("n")

    if len(dimension_strings) > 1:
        dimension_string = dimension_strings[1]
        dimension_indent = len(dimension_string) - len(dimension_string.lstrip())

        return "n" + " " * dimension_indent + non_nice_string

    return non_nice_string

np.core.arrayprint._array2string = _array2_nice_string

Outputs for:

some_fairly_long_named_arr = np.random.rand(2, 2)
print(f"{some_fairly_long_named_arr=}")

some_fairly_long_named_arr=array(
       [[0.95900608, 0.79367873],
       [0.58616975, 0.17757661]])

and

some_fairly_long_named_arr = np.random.rand(1, 2)
print(f"{some_fairly_long_named_arr=}")

some_fairly_long_named_arr=array([[0.62492772, 0.80453153]]).

I made it so if if the first dimension is 1, it is kept on the same line.

There is a non-internal method np.array2string that I tried to re-assign, but I never got that working. If someone could find a way to re-assign that public function instead of this internally used one, I’d imagine that’d make this solution a lot cleaner.

Answered By: Mario Ishac

Answer 2

I figured out a way to accomplish what I wanted, after reading through the CPython source:

import numpy as np
some_fairly_long_named_arr = np.random.rand(4, 2)
print(f"""{some_fairly_long_named_arr =
}""")

Which produces:

some_fairly_long_named_arr = 
array([[0.23560777, 0.96297907],
       [0.18882751, 0.40712246],
       [0.61351814, 0.1981144 ],
       [0.27115495, 0.72303859]])

I would rather prefer a solution that worked in a single line, but this seems to be the only way for now. Perhaps another way will be implemented in a later python version.

However note that the indentation on the continuation line has to be removed for the above mentioned method, as such:

    # ...some code with indentation...
    print(f"""{some_fairly_long_named_arr =
}""")
    # ...more code with indentation...

Otherwise, the alignment of the first line is broken again.

I tried using inspect.cleandoc and textwrap.dedent to alleviate this, but could not manage to fix the indentation issue. But perhaps this is the subject of another question.

Edit: After reading this article, I found a single line solution:

f_str_nl = lambda object: f"{chr(10) + str(object)}"  # add n directly
# f_str_nl = lambda object: f"{os.linesep + str(object)}"  # add rn on windows

print(f"{f_str_nl(some_fairly_long_named_arr) = !s}")

which outputs:

f_str_nl(some_fairly_long_named_arr) = 
[[0.26616956 0.59973262]
 [0.86601261 0.10119292]
 [0.94125617 0.9318651 ]
 [0.10401072 0.66893025]]

The only caveat is that the name of the object gets prepended by the name of the custom lambda function, f_str_nl.

I also found that a similar question was already asked here.

Answered By: Enigma Machine

Insert newline after equals sign in self documenting f-string in python3.8

Question:

Answers: