PDAL filter doesn't work: Unable to parse pipeline

So this is my first time to use PDAL. I use python 3.6 and PDAL 1.9.

 json_s = """{


        "test.las",
        {
        "type":"filters.outlier",
        "method":"statistical",
        "mean_k":12,
        "multiplier":0.5
        },
        {
        "type":"filters.range",
        "limits":"Classification![7:7]"
        },
        "testOut.las"

}"""

pipeline = pdal.Pipeline(json_s)
count = pipeline.execute()

It shows the err,

RuntimeError: JSON pipeline: Unable to parse pipeline. 

I checked the sample code in website and it looks the same. Just don't know why it doesn't work?

PDAL format look like this:

json_s = """{

 "pipeline":[
        "input.las",
        {
          #anything you need
        },

        "output.las"
       {
       }
    ]
}"""

In your case, try:

json_s = """{

 "pipeline":[
        "test.las",
        {
        "type":"filters.outlier",
        "method":"statistical",
        "mean_k":12,
        "multiplier":0.5
        },
        {
        "type":"filters.range",
        "limits":"Classification![7:7]"
        },
        "testOut.las"
    ]
}"""