This is a prime opportunity for keyword arguments.

```python
def __init__(self, real_time=0, ..., calib_e_fit=None, ...):
    """ define gamma spectra specific variables """    
    super(gammaSpec, self).__init__()
    self.real_time = real_time
    self.calib_e_fit = [] if calib_e_fit is None else calib_e_fit
```