the Eval function in Python

Question

Hello there i have the following code:

path = some destination on your harddrive

def K(path):
    try:

        getfile = open(path + '/test.txt')
        line = getfile.readlines()
        print line
        getfile.close()

    except:
        line = getfile.readlines()
        eval(line)
        d = dict()
        val= d[k]

to import a textfile, now my problem is to avoid the \n, which i assume can be done using the eval() function. I want to convert the string i get as input, to floats i can work with..

Thanx for any tips in advance

It would help if you describe the format of your text file. Using eval to avoid \n is something indescribable. — khachik
– khachik, Commented Dec 10, 2010 at 9:43
it takes some file.txt on my harddrive and imports the text to python shell as a string, so if the file for instant had 1,2 (newline) 2,3 (newline) 7,5 the input i get in shell is ['1,2\n', '2,3\n', '7,5'] and i want it to be numbers i can work with :) hope that helped — user457142
– user457142, Commented Dec 10, 2010 at 9:47
... So let's be clear: each line int the file has, as text, a comma-delimited list of integers on it, and you want to convert this text into a Python list of int objects? — Karl Knechtel
– Karl Knechtel, Commented Dec 10, 2010 at 12:48

SingleNegationElimination · Accepted Answer · 2010-12-10 09:43:51Z

2

Hint:

>>> float("\n1234\n")
1234.0

answered Dec 10, 2010 at 9:43

SingleNegationElimination

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khachik · Accepted Answer · 2010-12-10 09:54:54Z

1

I won't comment your code, just will post an example you can examine and modify to get it working. This function reads the content of a text file and converts tokens separated by whitespaces to floats if possible:

def getFloats(filepath):
  fd = open(filepath) # open the file
  try:
    content = fd.read().split() # read fully
    def flo(value):  # a function that returns a float for the given str or None
      try: return float(value)
      except ValueError: return None # skip invalid values
    # iterate through content and make items float or None,
    # iterate over the result to choose floats only
    return [x for x in [flo(y) for y in content] if x]
  finally:
    fd.close()

answered Dec 10, 2010 at 9:54

khachik

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Comments

6502 · Accepted Answer · 2010-12-10 15:22:16Z

1

your code is quite confused... to read a file that contains one float per line you can simply do:

val = map(float, open("test.txt"))

val will be a list containing your data with each element being a float

edited Dec 10, 2010 at 15:22

answered Dec 10, 2010 at 9:46

6502

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2 Comments

Ben Hoyt Over a year ago

val = [float(line) for line in open('test.txt')] would be both clearer and more memory-efficient (because iterating through lines in a file doesn't read all the lines at once, unlike readlines()).

6502 Over a year ago

@benhoyt: You're right... I often end up using L=open(xxx).readlines() when I need to play with many lines at the same time and it's something that dangerously got to the finger level now.

Ignacio Vazquez-Abrams · Accepted Answer · 2010-12-10 10:13:05Z

0

ast.literal_eval() will turn each line into tuples that you can then iterate or index for the values.

answered Dec 10, 2010 at 10:13

Ignacio Vazquez-Abrams

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Ben Hoyt · Accepted Answer · 2010-12-10 14:52:21Z

0

Here's a function read_numbers() which returns a list of lists of floats.

def read_numbers(filename):
    numbers = []
    with open(filename) as f:
        for line in f:
            lst = [float(word) for word in line.split(',')]
            numbers.append(lst)
    return numbers

If your file contains:

1, 2
2, 3
7, 5

Then read_numbers('filename') would return:

[[1.0, 2.0], [2.0, 3.0], [7.0, 5.0]]

You may want to do error handling (or simply ignore errors) by expanding out the inner list comprehension and wrapping the call to float() in a try ... except ValueError.

answered Dec 10, 2010 at 14:52

Ben Hoyt

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