Fix numpy.savetxt() TypeError: Mismatch between array dtype and format specifier (‘%.18e %.18e %.18e’) – NumPy Tutorial

By | October 17, 2019

When you are using numpy.savetxt() function to save numy array into a text file, you my get this error: TypeError: Mismatch between array dtype (‘<U31’) and format specifier (‘%.18e %.18e %.18e’). In this tutorial, we will introduce to you on how to fix this error.

Look at this example:

import numpy as np

data = np.array([[10, 20, 30], ['https://www.tutorialexample.com', 'python', 'programming']])
print(data)

fname= 'data.csv'
np.savetxt(fname, data)

In this example, we will save a 2-D numpy array into a csv file, Elements in numpy array are two types: integer and string.

Run this python script, we will get error like this:

numpy.savetxt() TypeError - Mismatch between array dtype and format specifier

Why this TypeError occurs?

As to numpy.savetxt() function.

numpy.savetxt(fname, X, fmt='%.18e', delimiter=' ', newline='n', header='', footer='', comments='# ', encoding=None)

The parameter fmt is %.18e defaultly. %.18e can format number (float, integer), however, it can not format string data.

To understand numpy.savetxt(), you can read this tutorial.

Understand numpy.savetxt() for Beginner with Examples – NumPy Tutorial

How to fix this TypeError?

We can use %s to format integer and string.

Here is an example.

fname= 'data.csv'
np.savetxt(fname, data, fmt = '%s')

Open data.csv, we will find content like:

10 20 30
https://www.tutorialexample.com python programming

Then this type error is fixed.

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