Using local variables with multiple assignments with pandas eval function

Question

The pandas help file says (for eval): As a convenience, multiple assignments can be performed by using a multi-line string.

However, I'm finding that doesn't work with variables (using ipython):

This works:

df_price.eval("op = op * @mult", inplace = True)

But this does NOT work (op, cl, hi, lo are cols in dataframe df_price where mult is a float):

df_price.eval("""op = op * @mult
              cl = cl * @mult
              hi = hi * @mult
              lo = lo * @mult""", inplace = True)

error: pandas.computation.ops.UndefinedVariableError: local variable 'mult' is not defined

I have submitted a pull request with a fix for this issue. See updated answer. — Stephen Rauch
– Stephen Rauch ♦, Commented Feb 8, 2017 at 6:07
The fix has been merged to main and is tagged as part of milestone 0.20.0. Cheers. — Stephen Rauch
– Stephen Rauch ♦, Commented Feb 9, 2017 at 22:08

Stephen Rauch · Accepted Answer · 2017-02-08 06:06:36Z

2

I can confirm that the local variables appear to only work on the first line of a multi line eval expression. A possible work around:

df_price.eval("""mult = @mult
              op = op * mult
              cl = cl * mult
              hi = hi * mult
              lo = lo * mult""", inplace = True)

This does however have the side effect of creating an extra column.

Update:

I have submitted a Pull Request with a fix for this issue.

edited Feb 8, 2017 at 6:06

answered Feb 7, 2017 at 8:47

Stephen Rauch♦

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3 Comments

techvslife Over a year ago

thanks. in that case, I'll do a string replace to replace mult with its value before passing the string to eval. Haven't investigated whether this is any faster than four separate eval statements anyway. The triple quote is clunky--I'd say a semi-colon to separate them is better, or allow a list: df_price.eval(["op = op * @mult", "cl = cl * @mult"], inplace=True)

Stephen Rauch Over a year ago

Your list example just needs a join added: df.eval("\n".join(..my_list..))

techvslife Over a year ago

Good point, that should work. However, I still think eval should be changed to not need the newline to process multiple commands--list separation is enough.

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