I recently discovered pandas "assign" method which I find very elegant. My issue is that the name of the new column is assigned as keyword, so it cannot have spaces or dashes in it.
df = DataFrame({'A': range(1, 11), 'B': np.random.randn(10)})
df.assign(ln_A = lambda x: np.log(x.A))
A B ln_A
0 1 0.426905 0.000000
1 2 -0.780949 0.693147
2 3 -0.418711 1.098612
3 4 -0.269708 1.386294
4 5 -0.274002 1.609438
5 6 -0.500792 1.791759
6 7 1.649697 1.945910
7 8 -1.495604 2.079442
8 9 0.549296 2.197225
9 10 -0.758542 2.302585
but what if I want to name the new column "ln(A)" for example? E.g.
df.assign(ln(A) = lambda x: np.log(x.A))
df.assign("ln(A)" = lambda x: np.log(x.A))
File "<ipython-input-7-de0da86dce68>", line 1
df.assign(ln(A) = lambda x: np.log(x.A))
SyntaxError: keyword can't be an expression
I know I could rename the column right after the .assign call, but I want to understand more about this method and its syntax.
You can pass the keyword arguments to
assign
as a dictionary, like so:assign
expects a bunch of key word arguments. It will, in turn, assign columns with the names of the key words. That's handy, but you can't pass an expression as the key word. This is spelled out by @EdChum in the comments with this linkuse
insert
instead for inplace transformationuse
concat
if you don't want inplace