Print statement with .format results in error - ipython

Question

I have following print statement:-
print("{0:0.2f}% in traing set".format((len(X_train)/(df.index))*100))
where
X_train = is 70% of sample data(training data)
and
df.index is RangeIndex(start=0, stop=768, step=1)
when I run print statement I get error as follow
non-empty format string passed to object.__format__
Answer of the print statement should be
69.92 in training set
30.08 in test set
I am not able to correct this behavior.
Any help will be appreciated.
Bharat.

Answer 1

I get this error if the object that's supposed to be formatted is a numpy array. RangeIndex() must be producing something similar.
In [82]: "{0:0.2f}% in traing set".format((20/(np.arange(3)))*100)
...
TypeError: non-empty format string passed to object.__format__
In [83]:
Same error if I just give it a list: "{0:0.2f}% in traing set".format([1,2,3])
Formatting works fine if the argument is just a number:
In [83]: "{0:0.2f}% in traing set".format((20/3)*100)
Out[83]: '666.67% in traing set'
The only format specifier that works with an object like a list or array is the plain str/repr,
In [102]: print("{} in traing set".format((len(X_train)/(np.arange(1,3)))*100))
[ 5000. 2500.] in traing set
===============
It's hard to read code in a comment; better to add it as an edit to the original question (clearly marked as such). Here's my guess as to what the formatting is:
value1 = len(X_train)
value2 = len(X_test)
value3 = len(df.index)
value4 = (value1/value3)*100
value5 = (value2/value3)*100
print("{0:0.2f}% in training set".format(value4))
print("{0:0.2f}% in test set".format(value5))
Yes, it is a good idea to separate the calculation of the values from the print formatting.