为什么我无法在同一Python块中同时捕获NameError和UnboundLocalError？

Question

A little background-- I have a Python program that makes plots from CSV files. I'm trying to make it more flexible by allowing a user to input between 1 and 3 files. I import the csv data into pandas data frames that are then used to generate a number of different plots.

I'm sure there's a better way to do what I'm doing, but I came across something strange (to me, at least) in my initial attempt to figure out if a user had input less than the maximum number of files.

Here is how I import the data:

# Imports the data.  The first two rows must be skipped due to the file format
data1 = pd.read_csv(filename1, skiprows=1, header=True)
if filename2 != '':
    data2 = pd.read_csv(filename2, skiprows=1, header=True)
if filename3 != '':
    data3 = pd.read_csv(filename3, skiprows=1, header=True)

So data2 and data3 are only defined if a user has provided a file name from my GUI. Later, I just wanted to use the existence of data2 and data3 to determine whether or not to plot the 2nd and 3rd data sets, respectively:

    try:
        axarr[1, 0].psd(data1[Ynew], NFFT=n_samples, Fs=fs, noverlap=n_overlap, window=mlab.window_hanning, label=baseFileName1)
    except KeyError:
        axarr[1, 0].psd(data1[Yold], NFFT=n_samples, Fs=fs, noverlap=n_overlap, window=mlab.window_hanning, label=baseFileName1)
    try:
        axarr[1, 0].psd(data2[Ynew], NFFT=n_samples, Fs=fs, noverlap=n_overlap, window=mlab.window_hanning, label=baseFileName2)
    except (UnboundLocalError, NameError):
        pass
    except KeyError:
        axarr[1, 0].psd(data2[Yold], NFFT=n_samples, Fs=fs, noverlap=n_overlap, window=mlab.window_hanning, label=baseFileName2)
    try:
        axarr[1, 0].psd(data3[Ynew], NFFT=n_samples, Fs=fs, noverlap=n_overlap, window=mlab.window_hanning, label=baseFileName3)
    except (UnboundLocalError, NameError):
        pass
    except KeyError:
        axarr[1, 0].psd(data3[Yold], NFFT=n_samples, Fs=fs, noverlap=n_overlap, window=mlab.window_hanning, label=baseFileName3)

And here's where it got weird. When I run the thing, it throws an UnboundLocalError telling me that 'NameError' is referenced before assignment. So that except block that's supposed to catch the UnboundLocalError isn't passing like I would expect. If I try to catch just the UnboundLocalError, a NameError is thrown. If I try to catch just a NameError, an UnboundLocalError is thrown. Can somebody explain to me what's going on here?

EDIT- Here's the traceback:

Error Traceback

Answer 1

I've revisited this multiple times and there seems to be bug report for this sort of problem floating around out there. Regardless, Exceptions are relatively expensive operations. You can use something like the following to circumvent your problem and cut down on cycles.

data1 = None
data2 = None
data3 = None
if filename1 != '':
    data1 = ...
if filename2 != '':
    data2 = ...
if filename3 != '':
    data3 = ...

if data1 is not None:
    if Ynew in data1:
        axarr[1,0].psd(data1[Ynew] ...
    elif Yold in data1:
        axarr[1,0].psd(data1[Yold] ...
if data2 is not None:
    if Ynew in data2:
        axarr[1,0].psd(data2[Ynew] ...
    elif Yold in data2:
        axarr[1,0].psd(data2[Yold] ...
if data3 is not None:
    if Ynew in data3:
        axarr[1,0].psd(data3[Ynew] ...
    elif Yold in data3:
        axarr[1,0].psd(data3[Yold] ...