Python – How to write a float list of lists to file in Python

file-iopython

I need to write a series of matrices out to a plain text file from python. All my matricies are in float format so the simple
file.write() and file.writelines()

do not work. Is there a conversion method I can employ that doesn't have me looping through all the lists (matrix = list of lists in my case) converting the individual values?

I guess I should clarify, that it needn't look like a matrix, just the associated values in an easy to parse list, as I will be reading in later. All on one line may actually make this easier!

Best Solution

m = [[1.1, 2.1, 3.1], [4.1, 5.1, 6.1], [7.1, 8.1, 9.1]]
file.write(str(m))

If you want more control over the format of each value:

def format(value):
    return "%.3f" % value

formatted = [[format(v) for v in r] for r in m]
file.write(str(formatted))

Related Solutions

Python – How to check if a list is empty

if not a:
  print("List is empty")

Using the implicit booleanness of the empty list is quite pythonic.

Python – How to check whether a file exists without exceptions

If the reason you're checking is so you can do something like if file_exists: open_it(), it's safer to use a try around the attempt to open it. Checking and then opening risks the file being deleted or moved or something between when you check and when you try to open it.

If you're not planning to open the file immediately, you can use os.path.isfile

Return True if path is an existing regular file. This follows symbolic links, so both islink() and isfile() can be true for the same path.

import os.path
os.path.isfile(fname)

if you need to be sure it's a file.

Starting with Python 3.4, the pathlib module offers an object-oriented approach (backported to pathlib2 in Python 2.7):

from pathlib import Path

my_file = Path("/path/to/file")
if my_file.is_file():
    # file exists

To check a directory, do:

if my_file.is_dir():
    # directory exists

To check whether a Path object exists independently of whether is it a file or directory, use exists():

if my_file.exists():
    # path exists

You can also use resolve(strict=True) in a try block:

try:
    my_abs_path = my_file.resolve(strict=True)
except FileNotFoundError:
    # doesn't exist
else:
    # exists

Best Solution

Related Solutions

Python – How to check if a list is empty

Python – How to check whether a file exists without exceptions

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