How can I read in my file line by line with each line as a list of floats?

Question

I have a file where each line has a pair of coordinates like so:

[-74.0104294, 40.6996416]

The code that I'm using to read them in is:

with open('Manhattan_Coords.txt', 'r') as f:
    mVerts = f.read().splitlines()

This reads in all 78 lines into a list, but it reads them in as strings, so when I print them out it shows up as:

['[(-74.0104294, 40.6996416]', ... , '[-74.0104294, 40.6996416]']

(Imagine the ... as 76 more coordinates like the first and last)

How can I read in each of these coordinate pairs as a list so that I am left with a list of 78 sublists with 2 floats inside each sublist?

Answer 1

For each line, you need to:

1. Read it
   ... using a for loop to iterate over the file
2. Strip whitespace including newlines
   ... using str.strip()
3. Lose the first and last characters (brackets)
   ... using string slicing: [1:-1]
4. Split on the substring ', '
   ... using str.split() and a list comprehension.
5. Convert the resulting strings into floats
   ... using float()
6. Add each pair of floats to a list
   ... using list.append()

That looks like this:

m_verts = []
with open('Manhattan_Coords.txt') as f:
    for line in f:
        pair = [float(s) for s in line.strip()[1:-1].split(", ")]
        m_verts.append(pair)

After which, m_verts looks like this:

>>> m_verts
[[-74.0104294, 40.6996416], ... ]

In general, you're better off iterating over the lines of a file than reading them all into a list at once with methods like splitlines() ... it's more readable, and with large files much more efficient.

Also, notice that I've used the more pythonic under_score style to name m_verts, rather than your camelCase style - and that there's no need to specify 'r' when opening a file for reading.

Answer 2

even better way than @ZeroPiraeus solution use ast.literal_eval which can evaluate any literal of python (here a list literal compound of float literal)

import ast
m_verts = []
with open('Manhattan_Coords.txt') as f:
    for line in f:
        pair = ast.literal_eval(line)
        m_verts.append(pair)

but for construction of the list even better is list comprehension

import ast
with open('Manhattan_Coords.txt') as f:
    m_verts = [ast.literal_eval(line) for line in f]