ValueError :Setting an array element with a sequence using numpy

Question

I have this piece of code in python

data = np.empty(temp.shape)
maxlat = temp.shape[0]
maxlon = temp.shape[1]
print(maxlat,maxlon)

for i in range(0,maxlat) :
    for j in range(0,maxlon):
        data[i][j] = p_temperature(pr,temp[i][j])

When I run this code in Python 3.5, I get this error

ValueError : setting an array element with a sequence

The value of maxlat is 181 and the value of maxlon is 360.

The shape of temp array is (181,360)

I also tried the suggestion in the comments:

for i in range(0,maxlat) :
    for j in range(0,maxlon):
        data[i][j] = temp[i][j]

But I get the same error.

Based on the error, it looks that p_temperator does not return a scalar, but a numpy array itself. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented May 20, 2017 at 11:48
@WillemVanOnsem - it just returns a number given two inputs p and temp — gansub
– gansub, Commented May 20, 2017 at 11:49
In the question. But please, just a sample (for example 5x5 not full 181x360), basically enough for us to reproduce your problem and help you solved it — Nuageux
– Nuageux, Commented May 20, 2017 at 12:04
@ADITYA No, that throws an ValueError: could not convert string to float: exception. :) — MSeifert
– MSeifert, Commented May 20, 2017 at 15:50

MSeifert · Accepted Answer · 2017-05-24 22:02:43Z

Based on the exception you get it seems likely that temp is an object array containing sequences. You could simply use numpy.empty_like:

data = np.empty_like(temp)  # instead of "data = np.empty(temp.shape)"

This creates a new empty array with the same shape and dtype - like your original array.

For example:

import numpy as np

temp = np.empty((181, 360), dtype=object)
for i in range(maxlat) :
    for j in range(maxlon):
        temp[i][j] = [1, 2, 3]

With the new approach it works:

data = np.empty_like(temp)
maxlat = temp.shape[0]
maxlon = temp.shape[1]
print(maxlat, maxlon)

for i in range(maxlat) :
    for j in range(maxlon):
        data[i][j] = temp[i][j]

And this temp array also reproduces the exception on your original code sample:

data = np.empty(temp.shape)  # your approach
maxlat = temp.shape[0]
maxlon = temp.shape[1]
print(maxlat, maxlon)

for i in range(maxlat) :
    for j in range(maxlon):
        data[i][j] = temp[i][j]

throws the exception:

ValueError: setting an array element with a sequence.

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