BUG: query modifies the frame when you compare with `=`

[TomAugspurger]

I messed up and used = instead of == in a query.

df = pd.DataFrame({'a': [1, 2, 3], 'b': ['a', 'b', 'c']})
df.query('a=1')

That raises a ValueError. But df was modified.

In [15]: df
Out[15]:
   a  b
0  1  a
1  1  b
2  1  c

versions:

pandas: 0.15.0-6-g403f38d
bottleneck: None
tables: None
numexpr: 2.3.1

Can't look right now.

[dxe4]

@TomAugspurger "a" is marked as "assigner" here https://github.com/pydata/pandas/blob/e463818e69edee64a277ce97c89999f0c898dd7c/pandas/computation/eval.py#L239 so env.target[parsed_expr.assigner] = ret does df['a'] = 1. Not sure if i can help further with this.

[dxe4]

it looks like eval is designed to mutate, but query is not meant to mutate. I can add an extra kwarg in eval indicating if you can mutate or not to fix this, if that makes sense. (query calls eval, eval mutates and returns None, so then query raises a value error)

[kohaugustine]

Hi @jorisvandenbossche , I'm here now on Github! I last communicated with you about this bug on Stackoverflow http://stackoverflow.com/questions/28714469/bug-in-pandas-query-method/ about the bug wherein the query() method would modify the data in a column if the query statement used an '=' symbol. I'd be keen to contribute to fixing this bug, however, if I do work on it, I can't guarantee to a strict timeline and may not have a lot of time to commit to it especially since at work I have moved on to a slightly different assignment where I am not doing much data analysis and thus not using pandas anymore (for now at least). I also have a fair amount of other projects outside of work that I have already undertaken. But I would say that I enjoy using python and am quite familar with it, so if solving this bug is within my reach, I definitely like to contribute to this !

[kohaugustine]

May I know what are the steps I need to take to be able to start assisting with this debug? Like do I have to clone the entire development repo and then create a branch or something? What's the flow like at the moment?

[jreback]

see docs here

• you fork the repo (only do this once)
• create a branch
• add tests which repro the problem
• make a fix
• tests should now pass
• submit a pull-request

[kohaugustine]

@jreback Thank you for pointing me to the docs and for describing the flow. I'll set to work on this, and if I have any issues, I'll raise them up to you guys.

[jreback]

closed by #11149