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Asked: 2026-06-01T13:00:35+00:00

I installed Enthought’s NumPy implementation for IronPython as per their instructions. Everything seems to

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I installed Enthought’s NumPy implementation for IronPython as per their instructions.

Everything seems to work, including import numpy and import scipy, but having these statements in an IronPython program causes the following exceptions:

A first chance exception of type 'System.Collections.Generic.KeyNotFoundException' occurred in IronPython.dll

A first chance exception of type 'System.MissingMemberException' occurred in Microsoft.Dynamic.dll

A first chance exception of type 'System.MissingMemberException' occurred in IronPython.dll

A first chance exception of type 'IronPython.Runtime.Exceptions.TypeErrorException' occurred in Snippets.debug.scripting

How do I fix these exceptions?

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  1. Editorial Team

    I wouldn’t worry about them, and I doubt they could be removed anyway. Those all map to Python exceptions, which means that they’re probably necessary to keep compatibility with CPython. In CPython, exceptions are fast (a couple orders of magnitude faster than .NET) so they’re much more commonly used in Python code than in C#.

    IronPython normally uses the .NET exception mechanism to surface Python exceptions, which can lead to huge slowdowns in some code. 2.7 add a “lightweight exception” mechanism, but it’s not consistently used (compatibility is more important than performance right now, but I would strongly consider a pull request). It’s possible that some of the code that throws these exceptions could be modified to use lightweight exceptions instead.

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