save numpy array in append mode

Is it possible to save a numpy array appending it to an already existing npy-file --- something like np.save(filename,arr,mode='a')?

I have several functions that have to iterate over the rows of a large array. I cannot create the array at once because of memory constrains. To avoid to create the rows over and over again, I wanted to create each row once and save it to file appending it to the previous row in the file. Later I could load the npy-file in mmap_mode, accessing the slices when needed.

The build-in .npy file format is perfectly fine for working with small datasets, without relying on external modules other then numpy.

However, when you start having large amounts of data, the use of a file format, such as HDF5, designed to handle such datasets, is to be preferred [1].

For instance, below is a solution to save numpy arrays in HDF5 with PyTables,

Step 1: Create an extendable EArray storage

import tables
import numpy as np

filename = 'outarray.h5'
ROW_SIZE = 100
NUM_COLUMNS = 200

f = tables.open_file(filename, mode='w')
atom = tables.Float64Atom()

array_c = f.create_earray(f.root, 'data', atom, (0, ROW_SIZE))

for idx in range(NUM_COLUMNS):
    x = np.random.rand(1, ROW_SIZE)
    array_c.append(x)
f.close()

Step 2: Append rows to an existing dataset (if needed)

f = tables.open_file(filename, mode='a')
f.root.data.append(x)

Step 3: Read back a subset of the data

f = tables.open_file(filename, mode='r')
print(f.root.data[1:10,2:20]) # e.g. read from disk only this part of the dataset

This is an expansion on Mohit Pandey's answer showing a full save / load example. It was tested using Python 3.6 and Numpy 1.11.3.

from pathlib import Path
import numpy as np
import os

p = Path('temp.npy')
with p.open('ab') as f:
    np.save(f, np.zeros(2))
    np.save(f, np.ones(2))

with p.open('rb') as f:
    fsz = os.fstat(f.fileno()).st_size
    out = np.load(f)
    while f.tell() < fsz:
        out = np.vstack((out, np.load(f)))

> out = array([[ 0., 0.], [ 1., 1.]])

I made a library to create Numpy .npy files that are larger than the main memory of the machine by appending on the zero axis. The file can then be read with mmap_mode="r".

https://pypi.org/project/npy-append-array

Installation

conda install -c conda-forge npy-append-array

or

pip install npy-append-array

Example

from npy_append_array import NpyAppendArray
import numpy as np

arr1 = np.array([[1,2],[3,4]])
arr2 = np.array([[1,2],[3,4],[5,6]])

filename = 'out.npy'

with NpyAppendArray(filename) as npaa:
    npaa.append(arr1)
    npaa.append(arr2)
    npaa.append(arr2)
    
data = np.load(filename, mmap_mode="r")

print(data)

Implementation Details

Appending to an array created by np.save might be possible under certain circumstances, since the .npy total header byte count is required to be evenly divisible by 64. Thus, there might be some spare space to grow the shape entry in the array descriptor. However, this is not guaranteed and might randomly fail. Initialize the array with NpyAppendArray(filename) directly (see above) so the header will be created with 64 byte of spare header space for growth.

Will 64 byte extra header space cover my needs?

It allows for up to 10^64 >= 2^212 array entries or data bits. Indeed, this is less than the number of atoms in the universe. However, fully populating such an array, due to limits imposed by quantum mechanics, would require more energy than would be needed to boil the oceans, compare

https://hbfs.wordpress.com/2009/02/10/to-boil-the-oceans

Therefore, a wide range of use cases should be coverable with this approach.

.npy files contain header which has shape and dtype of the array in it. If you know what your resulting array looks like, you can write header yourself and then data in chunks. E.g., here is the code for concatenating 2d matrices:

import numpy as np
import numpy.lib.format as fmt

def get_header(fnames):
    dtype = None
    shape_0 = 0
    shape_1 = None
    for i, fname in enumerate(fnames):
        m = np.load(fname, mmap_mode='r') # mmap so we read only header really fast
        if i == 0:
            dtype = m.dtype
            shape_1 = m.shape[1]
        else:
            assert m.dtype == dtype
            assert m.shape[1] == shape_1
        shape_0 += m.shape[0]
    return {'descr': fmt.dtype_to_descr(dtype), 'fortran_order': False, 'shape': (shape_0, shape_1)}

def concatenate(res_fname, input_fnames):
    header = get_header(input_fnames)
    with open(res_fname, 'wb') as f:
        fmt.write_array_header_2_0(f, header)
        for fname in input_fnames:
            m = np.load(fname)
            f.write(m.tostring('C'))

If you need a more general solution (edit header in place while appending) you'll have to resort to fseek tricks like in [1].

Inspired by
[1]: https://mail.scipy.org/pipermail/numpy-discussion/2009-August/044570.html (doesn't work out of the box)
[2]: https://docs.scipy.org/doc/numpy/neps/npy-format.html
[3]: https://github.com/numpy/numpy/blob/master/numpy/lib/format.py

For appending data to an already existing file using numpy.save, we should use:

f_handle = file(filename, 'a')
numpy.save(f_handle, arr)
f_handle.close()

I have checked that it works in python 2.7 and numpy 1.10.4

I have adapted the code from here, which talks about savetxt method.

you can try something like reading the file then add new data

import numpy as np
import os.path

x = np.arange(10) #[0 1 2 3 4 5 6 7 8 9]

y = np.load("save.npy") if os.path.isfile("save.npy") else [] #get data if exist
np.save("save.npy",np.append(y,x)) #save the new

after 2 operation:

print(np.load("save.npy")) #[0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9]

The following is based upon PaxRomana99's answer. It creates a class that you can use to save and load the arrays. Ideally, one would also change the header of the npy file every time a new array is added in order to modify the description of the shape (see here for the description of the header)

import numpy as np
import pickle

from pathlib import Path
import os


class npyAppendableFile():
    def __init__(self, fname, newfile=True):
        '''
        Creates a new instance of the appendable filetype
        If newfile is True, recreate the file even if already exists
        '''
        self.fname=Path(fname)
        if newfile:
            with open(self.fname, "wb") as fh:
                fh.close()
        
    def write(self, data):
        '''
        append a new array to the file
        note that this will not change the header
        '''
        with open(self.fname, "ab") as fh:
            np.save(fh, data)
            
    def load(self, axis=2):
        '''
        Load the whole file, returning all the arrays that were consecutively
        saved on top of each other
        axis defines how the arrays should be concatenated
        '''
        
        with open(self.fname, "rb") as fh:
            fsz = os.fstat(fh.fileno()).st_size
            out = np.load(fh)
            while fh.tell() < fsz:
                out = np.concatenate((out, np.load(fh)), axis=axis)
            
        return out
    
    
    def update_content(self):
        '''
        '''
        content = self.load()
        with open(self.fname, "wb") as fh:
            np.save(fh, content)

    @property
    def _dtype(self):
        return self.load().dtype

    @property
    def _actual_shape(self):
        return self.load().shape
    
    @property
    def header(self):
        '''
        Reads the header of the npy file
        '''
        with open(self.fname, "rb") as fh:
            version = np.lib.format.read_magic(fh)
            shape, fortran, dtype = np.lib.format._read_array_header(fh, version)
        
        return version, {'descr': dtype,
                         'fortran_order' : fortran,
                         'shape' : shape}
                
        
      
arr_a = np.random.rand(5,40,10)
arr_b = np.random.rand(5,40,7)    
arr_c = np.random.rand(5,40,3)    

f = npyAppendableFile("testfile.npy", True)        

f.write(arr_a)
f.write(arr_b)
f.write(arr_c)

out = f.load()

print (f.header)
print (f._actual_shape)

# after update we can load with regular np.load()
f.update_content()


new_content = np.load('testfile.npy')
print (new_content.shape)