Insert newline after equals sign in self documenting f-string

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:

>>> print(f"some_fairly_long_named_arr=\n{some_fairly_long_named_arr!r}")
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 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.

Try checking the relevant PEP or issue. If it's not there, the answer is probably not. Then check the CPython source. If it's not there, the answer is definitely not. — Mateen Ulhaq, May 09 '20 at 10:19
The [relevant issue](https://bugs.python.org/issue36817) does not seem to contain my use case. I will check the [CPython source](https://github.com/python/cpython/commit/9a4135e939bc223f592045a38e0f927ba170da32) at a more convenient time. — Enigma Machine, May 10 '20 at 07:54

score 0 · Answer 1 · edited May 24 '23 at 18:43

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.

My god, this is far more complex than I thought it would be :) However, I would rather prefer a solution that works for many data types in general: numpy arrays, dataframes, etc... I will edit the question. — Enigma Machine, May 09 '20 at 11:32
That indentation is not correct so the output not properly aligned. I think you actually want something more like `dimension_indent = len('array(')`. — wjandrea, May 24 '23 at 18:49

score 0 · Accepted Answer · edited May 24 '23 at 18:59

Multi-line solution

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.

Single-line solution

I found this after reading this article:

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

Insert newline after equals sign in self documenting f-string

2 Answers2

Multi-line solution

Single-line solution

Linked