Changing strings to Floats in an imported .csv

问题:

Quick question for an issue I haven't managed to solve quickly:

I'm working with a .csv file and can't seem to find a simple way to convert strings to floats. Here's my code,

import csv

def readLines():
    with open('testdata.csv', 'rU') as data:
        reader = csv.reader(data)
        row = list(reader)
        for x in row:
            for y in x:
                print type(float(y)),
readLines()

As you can see, it will currently print the type of every y element in x set of lists in the variable row; this produces a long list of "<type 'float'>". But this doesn't actually change each element to a float, nor does setting the for loop to execute float(y) (a type test returns 'string' for each element) work either.

I also tried literal_eval, but that failed as well. The only way to change the list elements to floats is to create a new list, either with list comprehension or manually, but that loses the original formatting of each list (as lists of a set amount of elements within one larger list).

I suppose the overall question is really just "What's the easiest way to read, organize, and synthesize data in .csv or excel format using Python?"

Thanks in advance to those courteous/knowledgeable enough to help.

回答1:

You are correct that Python's builtin csv module is very primitive at handling mixed data-types, does all its type conversion at import-time, and even at that has a very restrictive menu of options, which will mangle most real-world datasets (inconsistent quoting and escaping, missing or incomplete values in Booleans and factors, mismatched Unicode encoding resulting in phantom quote or escape characters inside fields, incomplete lines will cause exception). Fixing csv import is one of countless benefits of pandas. So, your ultimate answer is indeed stop using builtin csv import and start using pandas. But let's start with the literal answer to your question.

First you asked "How to convert strings to floats, on csv import". The answer to that is to open the csv.reader(..., quoting=csv.QUOTE_NONNUMERIC) as per the csv doc

csv.QUOTE_NONNUMERIC: Instructs the reader to convert all non-quoted fields to type float.

That works if you're ok with all unquoted fields (integer, float, text, Boolean etc.) being converted to float, which is generally a bad idea for many reasons (missing or NA values in Booleans or factors will get silently squelched). Moreover it will fail (throw exception) on unquoted text fields obviously. So it's brittle and needs to be protected with try..catch.

Then you asked: 'I suppose the overall question is really just "What's the easiest way to read, organize, and synthesize data in .csv or excel format using Python?"' to which the crappy csv.reader solution is to open with csv.reader(..., quoting=csv.QUOTE_NONNUMERIC)

But as @geoffspear correctly replied 'The answer to your "overall question" may be "Pandas", although it's a bit vague.'

回答2:

When converting a bunch of strings to floats, you should use a try/except to catch errors:

def conv(s):
    try:
        s=float(s)
    except ValueError:
        pass    
    return s

print [conv(s) for s in ['1.1','bls','1','nan', 'not a float']] 
# [1.1, 'bls', 1.0, nan, 'not a float']

Notice that the strings that cannot be converted are simply passed through unchanged.

A csv file IS a text file, so you should use a similar functionality:

def readLines():
    def conv(s):
        try:
            s=float(s)
        except ValueError:
            pass    
        return s

    with open('testdata.csv', 'rU') as data:
        reader = csv.reader(data)
        for row in reader:
            for cell in row:
                y=conv(cell)
              # do what ever with the single float
         # OR
         # yield [conv(cell) for cell in row]  if you want to write a generator...    

回答3:

Try something like the following

import csv

def read_lines():
    with open('testdata.csv', 'rU') as data:
        reader = csv.reader(data)
        for row in reader:
            yield [ float(i) for i in row ]

for i in read_lines():
    print(i)

# to get a list, instead of a generator, use
xy = list(read_lines())

As for the easiest way, then I suggest you see the xlrd, xlwt modules, personally I always have hard time with all the varying CSV formats.

回答4:

for y in x:
                print type(float(y)),

float(y) takes the value of y and returns a float based on it. It does not modify y- it returns a new object.

y = float(y)

is more like what you are looking for- you have to modify the objects.