Hello World issues when using %% with ipython

[landlord1984]

In [44]: %%python3 hello.py
    ...: 
UsageError: %%python3 is a cell magic, but the cell body is empty.

In [45]: %%python3 hello.py
    ...: d
    ...: 
Hello World

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Above is what displayed on ipython.

It is weird I type"%%python3 hello.py" and ENTER, error message comes out.
But, if I typed something, e.g. "d" after ENTER, and then ENTER again, my "Hello World" is printed from hello.py.

Why? Thanks.

L