Appending array rows to .txt file in Python 3

Question

I am simply trying to put my 11-column, single row of values into a text file that I can append new rows to. I am used to python 2.7 and I do not understand why my np.savetxt is not behaving the same as previously.

I know that this will create my text file without having any type errors:

with open("testtext.txt", "wb") as myfile:
    np.savetxt(myfile, mydata, fmt="%.5f", delimiter=' ', newline=' ')

However when I then run my code to receive my new data that I need to append, and I change it to:

with open("testtext.txt", "ab") as myfile:
    np.savetxt(myfile, mynewdata, fmt="%.5f", delimiter=' ', newline=' ')

It does not put mynewdata into a new row. I have input \n into newline, I have tried " " instead of ' ', I have tried \r\n but that was a longshot.. Please help, I feel like this should be a simple thing. Thank you.

Will · Accepted Answer · 2017-01-13 08:29:51Z

4

Going through your question again, so, if your new array(row) not started in a new row, that should be the last line of your text file was not ended with newline. So as @hpaulj pointed out, this should work:

with open("testtext.txt", "wb") as myfile:
    f.write(b'\n')
    ...

If you are running on windows, take '\r\n' instead of '\n'.

edited Jan 13, 2017 at 8:29

answered Jan 13, 2017 at 4:12

Will

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1 Comment

layces Over a year ago

Thank you @Will, this worked and I did have to use '\r\n'!

hpaulj · Accepted Answer · 2017-01-13 07:56:01Z

If you save a 1d array with typical delimiter and no special newline, it saves one value per row:

In [548]: with open('test.txt','wb') as f:
     ...:     np.savetxt(f, np.arange(3), fmt='%5d', delimiter=',')   
In [549]: cat test.txt
    0
    1
    2

Using '' newline, puts them on one line - but ends the file without a \n

In [550]: with open('test.txt','wb') as f:
     ...:     np.savetxt(f, np.arange(3), fmt='%5d', delimiter=',', newline='')

In [552]: cat test.txt
    0    1    2In [553]:

The next append will continue on that line.

I could include a b'\n' write. This needs to be a bytestring nl. (np.savetxt makes liberal use of np.lib.npyio.asbytes).

In [557]: with open('test.txt','ab') as f:
     ...:     f.write(b'\n')
     ...:     np.savetxt(f, np.arange(3), fmt='%5d', delimiter=',', newline='')

In [558]: cat test.txt
    0    1    2
    0    1    2In [559]:

Or maybe I should make a habit of adding a \n write at the end of such a savetxt.

OR, I could just make sure I write a 2d array to the file, even if I want to write just one line. Wrapping the array in a list will do the trick. Think of savetxt iterating on the 'rows' (first dimension) of the input,and writing one row at a time (formatted with fmt, delimiter, and newline).

In [559]: with open('test.txt','wb') as f:
     ...:     np.savetxt(f, [np.arange(3)], fmt='%5d', delimiter=',')

In [560]: cat test.txt
    0,    1,    2

In [561]: with open('test.txt','ab') as f:
     ...:     np.savetxt(f, [np.arange(4)], fmt='%5d', delimiter=',')

In [562]: cat test.txt
    0,    1,    2
    0,    1,    2,    3

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