How to create converters for pandas?-CodePudding

I want to read in a CSV file with pandas. Two columns are to be multiplied by a different constant each.

params = {
    "table-columns": ['A', 'B'],
    "multiplicands": ['0.001', '0.000001']
}

converters_ = {}
for i in range(0,len(params['table-columns'])):
    column = params['table-columns'][i]
    multiplicand = params['multiplicands'][i]
    display(column   '/'   multiplicand)
    func = lambda x: float(x) * float(multiplicand)
    converters_[column] = func
display(converters_['A'](1)) # should be 1e-03
display(converters_['B'](1)) # should be 1e-06

#df = pd.read_csv('data.csv', converters=converters_)

This is the output:

'A/0.001'
'B/0.000001'
1e-06
1e-06

Column A should be multiplied by 0.001, but it is multiplied by 0.000001. Where is the bug here?

CodePudding user response：

The issue is that func, which you've stored in converters_[0] and converters_[1], does not evaluate multiplicand until you actually call func, at which point multiplicand is "0.000001".

You can try replacing the assignment to func with this call to eval() which hardcodes the value of multiplicand into the body of the lambda:

    func = eval(f"lambda x: float(x) * {float(multiplicand)}")

Output:

A/0.001
B/0.000001
0.001
1e-06