Pandas df.eval fails for Unary operation involving float32 column

Code Sample, a copy-pastable example if possible

import pandas as pd

df = pd.DataFrame({
    'Foo64': [1.2, 2.4, 7.24],
})

df['Foo32'] = df['Foo64'].astype('float32')

df.eval('-1 + Foo64')   # Works
df.eval('-1 + Foo32')   # Throws Exception

```python traceback Traceback (most recent call last): File "C:\Users\vmuriart\Desktop\bug_eval.py", line 10, in df.eval('-1 + Foo32') # Throws Exception File "C:\Anaconda\lib\site-packages\pandas\core\frame.py", line 2186, in eval return _eval(expr, inplace=inplace, kwargs) File "C:\Anaconda\lib\site-packages\pandas\core\computation\eval.py", line 262, in eval truediv=truediv) File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 727, in init self.terms = self.parse() File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 744, in parse return self._visitor.visit(self.expr) File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 313, in visit return visitor(node, kwargs) File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 319, in visit_Module return self.visit(expr, kwargs) File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 313, in visit return visitor(node, kwargs) File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 322, in visit_Expr return self.visit(node.value, kwargs) File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 313, in visit return visitor(node, kwargs) File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 417, in visit_BinOp left, right = self._maybe_downcast_constants(left, right) File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 365, in _maybe_downcast_constants name = self.env.add_tmp(np.float32(left.value)) AttributeError: 'UnaryOp' object has no attribute 'value'

```

Problem description

df.eval(...) operations fail if one of the columns involved is of type float32 and some unary operations are involved. In the example above df.eval('-1 + Foo32') failed, but df.eval('-Foo32') will succeed, and df.eval('-1*Foo32') will also fail.

Originally I tested fixing this by just adding self.value = operand.value to the UnaryOp object, before I realized that this issue only affects float32 objects. I haven't looked to see where the code branches off to cause this misbehavior.

This was tested and reproduced on both 0.20.3 and 0.22.0

Expected Output

No exception thrown.

Output of pd.show_versions()

Comment From: TomAugspurger

Looks like a duplicate of https://github.com/pandas-dev/pandas/issues/16363. Some more details there if you want to try submitting a fix.

Comment From: vmuriart

Thanks! didn't find it this morning, though I only searched from my phone... thanks!