Python. ValueError could not convert string to float:

Question

I'm trying to produce the average of numbers in a specified column in a text file. I am receiving an error that python could not convert string to float, although I don't see where I could be passing it an invalid string.

def avg_col(f, col, delim=None, nhr=0):
    """
    file, int, str, int -> float

    Produces average of data stored in column col of file f

    Requires: file has nhr header rows of data; data is separated by delim

    >>> test_file = StringIO('0.0, 3.5, 2.0, 5.8, 2.1')
    >>> avg_col(test_file, 2, ',', 0)
    2.0

    >>> test_file = StringIO('0.0, 3.5, 2.0, 5.8, 2.1')
    >>> avg_col(test_file, 3, ',', 0)
    5.8
    """
    total = 0 
    count = 0


    skip_rows(f, nhr)
    for line in f: 
        if line.strip() != '':
            data = line.split(delim)
            col_data = data[col]
            total = sum_data(col_data) + total
            count = len(col_data) + count 
    return total / count

def sum_data(lod):
    '''
    (listof str) -> Real 

    Consume a list of string-data in a file and produce the sum

    >>> sum_data(['0'])
    0.0
    >>> sum_data(['1.5'])
    1.5

    '''
    data_sum = 0
    for number in lod: 
        data_sum = data_sum + float(number)
    return data_sum

You need to share the full traceback, and if you can provide some sample data that reproduces the exception, that'd be far more helpful than just the code you posted. — Martijn Pieters, Jan 26 '15 at 20:24

score 2 · Accepted Answer · answered Jan 26 '15 at 20:28

You are passing in one string to sum_lod():

data = line.split(delim)
col_data = data[col]
total = sum_data(col_data) + total

data is a list of strings, data[col] is then one element.

sum_data() on the other hand expects an iterable:

def sum_data(lod):
    # ...
    for number in lod:

Iterating over a number then gives you the individual characters:

>>> for element in '5.8':
...     print element
... 
5
.
8

Trying to turn each element of such a string can easily lead to you trying to convert characters that are not numbers to a float:

>>> float('.')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float: .

Either pass in a list of strings:

total += sum_data([col_data])
count += 1

or simply use float() on the one element you have:

total += float(col_data)
count += 1

Python. ValueError could not convert string to float:

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