Why we cannot tranfer adata along with functions?

Hello all,
For these 2 functions,
sc.pp.filter_cells(adata, min_genes=200)
sc.pp.filter_genes(adata, min_cells=3)
the authors make inplace=True as default.

Because I want to tranfer the output into an variable, I change these functions to
a=sc.pp.filter_cells(adata, min_genes=200, inplace=False)
sc.pp.filter_genes(a, min_cells=3, inplace=False),
but it creates errors and the output of a is NoType:
aceback (most recent call last):
File “C:\Users\Yuanjian\AppData\Local\Programs\Python\Python36\lib\site-packages\IPython\core\interactiveshell.py”, line 3343, in run_code
exec(code_obj, self.user_global_ns, self.user_ns)
File “”, line 2, in
sc.pp.filter_genes(a, min_cells=3, inplace=False) # exclude genes only expressed in <3 cells
File “C:\Users\Yuanjian\AppData\Local\Programs\Python\Python36\lib\site-packages\scanpy\preprocessing_simple.py”, line 259, in filter_genes
X if min_cells is None and max_cells is None else X > 0, axis=0

Does anybody know why inplace=False doesn’t work?