marginal_counts() raises TypeError when meas_qubits is default

[ryan-woo]

Informations

• Qiskit Ignis version: 0.3.0
• Python version: 3.6.4
• Operating system: MacOS

What is the current behavior?

When using the marginal_counts() function in qiskit.ignis.verification.marginal_counts() with default meas_qubits=True, then the function raises a TypeError here.

https://github.com/Qiskit/qiskit-ignis/blob/master/qiskit/ignis/verification/tomography/data.py#L60

Steps to reproduce the problem

>>> marginal_counts(counts)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/rwoo/rwoo/lib/python3.6/site-packages/qiskit/ignis/verification/tomography/data.py", line 60, in marginal_counts
    if num_qubits == len(meas_qubits) or (meas_qubits is True):
TypeError: object of type 'bool' has no len()

What is the expected behavior?

marginal_counts() returns all of the counts trimmed of any whitespace.

Suggested solutions

Reorder the if condition to check that meas_qubits == True before trying to do list operations on it.