Patsy prints uninformative error message when user places "Intercept" in a formula

[setjmp]

This is observed on patsy 0.1.0.

I saw that the design_info object of a design matrix uses "Intercept" as the encoding for the intercept term so I wondered what would happen if a programmer chose this as the name for a feature.

The ideal scenerio is that patsy either:
a. Does some name mangling
b. throws an error telling me exactly what I did wrong if this is not going to be supported

What happens in reality is that an uniformative assertion message is produced:

Traceback (most recent call last): File "failure.py", line 5, in <module> y,X = patsy.dmatrices("sl ~ Intercept",dataFrame) File "build/bdist.macosx-10.8-intel/egg/patsy/highlevel.py", line 283, in dmatrices File "build/bdist.macosx-10.8-intel/egg/patsy/highlevel.py", line 150, in _do_highlevel_design File "build/bdist.macosx-10.8-intel/egg/patsy/build.py", line 860, in build_design_matrices File "build/bdist.macosx-10.8-intel/egg/patsy/build.py", line 776, in _build File "build/bdist.macosx-10.8-intel/egg/patsy/build.py", line 757, in design_info File "build/bdist.macosx-10.8-intel/egg/patsy/design_info.py", line 78, in __init__ AssertionError

Here is the code that produces the error:

import pandas
import patsy

dataFrame = pandas.io.parsers.read_csv("salary2.txt") 
y,X = patsy.dmatrices("sl ~ Intercept",dataFrame) 

[njsmith]

Oo, sneaky.

Yeah, we should probably do some name mangling, since the same thing could happen when people create custom factor objects. Maybe I can re-use the name mangling for the automatic name creation (i.e. just say that unnamed columns are called "x" and then let the name mangler turn that into "x1", "x2", ...).