Get information about a function in python, looking at source code

Question

Hi,

the following code comes from the matplotlib gallery:

#!/usr/bin/env python
from pylab import *

x = array([10, 8, 13, 9, 11, 14, 6, 4, 12, 7, 5])
y = array([8.04, 6.95, 7.58, 8.81, 8.33, 9.96, 7.24, 4.26, 10.84, 4.82, 5.68])

I am new to python, and would like to change the content of x and y from an input file. I have two short questions:

1. I could guess what array means, but once I see it on the code, how can I know to which library it belongs and more information about it? Should I use some kind of python debug commands?
2. How do I insert the content of my input file into x?

Thanks

Answer 1

array appears to be a function/class in pylab. You could do help(array) to find out more - this relies on the presence of a proper docstring for array.

from module import * is a problematic idiom in several ways precisely for the reason that it makes it hard to find out where identifiers come from.

import pylab
x = pylab.array(...)

might be a better way.

Answer 2

As to your 1. that's due to bad habits of the person giving you that program. It should have been:

#!/usr/bin/env python
import pylab

x = pylab.array([10, 8, 13, 9, 11, 14, 6, 4, 12, 7, 5])
y = pylab.array([8.04, 6.95, 7.58, 8.81, 8.33, 9.96, 7.24, 4.26, 10.84, 4.82, 5.68])

help(pylab.array)

or

#!/usr/bin/env python
from pylab import array

x = array([10, 8, 13, 9, 11, 14, 6, 4, 12, 7, 5])
y = array([8.04, 6.95, 7.58, 8.81, 8.33, 9.96, 7.24, 4.26, 10.84, 4.82, 5.68])

help(array)

Use the help(something). It helps :)

Using explicit imports is really handy if programs become a bit more complex. The only case I know where from package import * is nice is when playing at the python prompt, trying stuff out.

As to the file, can you sketch how the file looks like? And I believe numpy has an array load function somewhere, look at the cookbook.

Answer 3

(1) Fire up the python console and use the dir() function on whatever object/class/variable you want to know more about. It'll print out all methods and properties of the object.

>> import pylab        
>> x =  pylab.array([10, 8, 13, 9, 11, 14, 6, 4, 12, 7, 5])
>> dir(x)

(2) look up File Objects in the python documentation

Answer 4

• Use the help function to find out what an object can do.

  import pylab
  help(pylab.array)
  

• "What library it belongs to" is a sort of funny question. You can easily know what module you are using it from if you refactor your code not to use import *. (Never ever ever use import *.) That does not mean that's it's original home—pylab is just a place where a bunch of stuff is pooled for convenience. pylab gets it from matplotlib.pylab which gets it from numpy which gets it from numpy.core.multiarray (which is a C extension module). You can see its original home by looking at pylab.array.__module__ when it's important, which is not often.

  • Personally, I think you're better of not using pylab at all, just getting stuff from numpy, scipy, and matplotlib as you need it to keep things organized. To quote The Zen of Python: Namespaces are one honking great idea -- let's do more of those!

• How to build an numpy array from a file depends on the format of the file. numpy.fromfile/pylab.fromfile can import a binary file storing the array's data. (This is the same format used by numpy.tofile—big shock there.)

Answer 5

Replying to question #2, if your file looks like this:

10 8.04
8 6.95
13 7.58
9 8.81
11 8.33
14 9.96
6 7.24
4 4.26
12 10.84
7 4.82
5 5.68

Then I would do like this:

x_list = list()
y_list = list()

fp = open('coords.txt', 'r')
for line in fp:
    line = line.strip()   # Remove trailing whitespace
    if not line: continue # Skip empty lines
    a,b = line.split()    # Split line by whitespace, storing coords in a & b
    x_list.append(a)      # Add a to x_list
    y_list.append(b)      # Add b to y_list
fp.close()

x = array(x_list)
y = array(y_list)