Print content and several attributes of a numpy array to a text file

Question

I have an array. My task is to print out the array, along with its shape, size, item size, dimensions, and data type name. The output should be a text file - each attribute should be on a new line.

When I attempt to use the following code, I get the error:

  File "<ipython-input-76-f4d4f45285be>", line 1, in <module>
    print(a.shape)

AttributeError: 'NoneType' object has no attribute 'shape'

I have tried two options, open a text file and np.savetxt. Neither seems to work.

Here is the code:

import numpy as np

a = np.arange(15).reshape(3,5)

a = print(a)

shape = print(a.shape)

size = print(a.size)

itemsize = print(a.itemsize)

ndim = print(a.ndim)

dtype = print(type(a.dtype))

with open("demo_numpy.tx","w") as text:
    text.write(a,shape,size,itemsize,ndim,dtype, file = text)

np.savetxt('demo_numpy.txt',[a,shape,size,itemsize,ndim,dtype])

What am I doing wrong, and how can I fix my output?

Any particular reason why you're doing a = print(a)? print returns None. — cs95
– cs95, Commented Jul 15, 2017 at 14:45
I added the "variable =" so that I could list the print statements in the output file statement — zsad512
– zsad512, Commented Jul 15, 2017 at 14:48
Ah, you're mistaken about what print does. MSeifert explains why. — cs95
– cs95, Commented Jul 15, 2017 at 14:48

MSeifert · Accepted Answer · 2017-07-15 14:57:41Z

1

print just prints the value passed in to stdout and returns None. If you want to access a property just do it without print:

import numpy as np

a = np.arange(15).reshape(3,5)
shape = a.shape
size = a.size
itemsize = a.itemsize
ndim = a.ndim
dtype = a.dtype

And if you want to print don't assign the return value of print:

print(a)
print(a.shape)
print(a.size)
print(a.itemsize)
print(a.ndim)
print(a.dtype)

Note that you don't correctly write to files, in the first case you can only write one argument at a time, you need to either str.join them or do multiple text.writes. In the second case you should check the documentation of numpy.savetxt - it expects an array as second argument not a list of several attributes.

For example:

with open("demo_numpy.tx","w") as text:
    text.write(str(a))
    text.write(str(shape))
    text.write(str(size))
    text.write(str(itemsize))
    text.write(str(ndim))
    text.write(str(dtype))

# or:
#   text.write('\n'.join(map(str, [a,shape,size,itemsize,ndim,dtype])))

np.savetxt('demo_numpy.txt', a)

edited Jul 15, 2017 at 14:57

answered Jul 15, 2017 at 14:45

MSeifert

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6 Comments

zsad512 Over a year ago

When I try your suggestion I get two errors. One with each text function. TypeError: write() takes no keyword arguments and TypeError: Mismatch between array dtype ('object') and format specifier ('%.18e')

MSeifert Over a year ago

The last paragraph should explain why that happens. You're using file.write and np.savetxt incorrectly. I.e. try np.savetxt('demo_numpy.txt', a) instead.

zsad512 Over a year ago

Ok, so I want to access the values as per the first code block in your comment. But now, how do I print them out to a txt file with each variable on a new line?

MSeifert Over a year ago

Well, just text.write them one at a time and insert newlines when appropriate. :)

zsad512 Over a year ago

@MSeifert- just for clarification. I need to print the data type name, and not the datatype. Have I done this correctly? What is the difference?

|

Nriuam · Accepted Answer · 2017-07-15 15:23:06Z

0

I'd like to use something like this:

# import numpy as np
# my_array = np.arange(3)
metadata = [(method, getattr(my_array, method)) for method in dir(my_array) if (not callable(getattr(my_array, method))) and (not method.startswith('__'))]
names, values = zip(*metadata)  # 2 lists

Then loop over names & values and write into a file.

answered Jul 15, 2017 at 15:23

Nriuam

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