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NumPy | array2string method

schedule May 21, 2023
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Numpy's array2string(~) method returns a string representation of your Numpy array formatted as specified.

This method defaults to using the configuration specified via Numpy's get_printoptions(~). You can change the default configuration using set_printoptions(~).


1. a | array-like

The input array. Unlike most Numpy methods, a has to be a Numpy array.

2. max_line_widthlink | int | optional

The maximum number of characters per line. By default, max_line_width=75, unless overridden via set_printoptions(~).

3. precisionlink | int or None | optional

The number of decimal places to show. A precision of 3 would mean 3.1415 becomes 3.142. If None is passed and floatmode is not fixed, then the precision will be such that the values are uniquely differentiated. By default, precision=8 unless overridden.

4. suppress_smalllink | boolean | optional

Whether to show values in full decimals instead of using scientific notation. This is only applicable for floats whose absolute values are smaller than 1e-4, or the ratio between the largest value and the smallest value in the array is larger than 1000. By default, suppress_small=False unless overridden.

5. separatorlink | string | optional

The string to separate the values. By default, separator=" " (i.e. a single whitespace).

6. prefixlink | string | optional

See the explanation of the suffix parameter below.

7. suffixlink | string | optional

Parameters prefix and suffix are used to add padding to each line. Each line can only hold a maximum of max_line_width-len(prefix)-len(suffix) characters. Note that the length of prefix and suffix matters, not their actual value since they are not printed out.

8. formatterlink | dict<string,function> | optional

The mapping to apply to different data-types. The dictionary's key-value pair is as follows:

  • key: the type you wish to apply a mapping on

  • value: a function that takes as input the value with type key, and returns a new value.

Here are some of main data-types:




Convert booleans.


Convert integers.


Convert floats.


Convert timedeltas.


Convert datetimes.

Here are some special keys that you can provide:




Convert all data-types.


Convert "float" and "longfloat"


Convert "str" and "numpystr"

By default, formatter=None unless overridden.

9. thresholdlink | int | optional

If the number of values in the array is larger than threshold, then instead of obtaining each and every values, the values will be truncated with .... By default, threshold=1000 unless overridden.

10. edgeitemslink | int | optional

If truncation occurs, then the number of values to show in the front and back. By default, edgeitems=3 unless overridden.

11. signlink | string | optional

How to handle the sign of the values:




Omits the +.


Places a + in front of positive numbers.

" "

Places a single white space in front of positive numbers.

By default, sign="-" unless overridden.

10. floatmodelink | string | optional

How to handle precision for floats:




Always show the specified precision. This results in all floats having the same decimal place.


Show minimum number of decimal places so as to uniquely identify the values. This ignores the specified precision.


Prints at most the specified precision.


Prints at most the specified precision, and also ensures that all floats have the same decimal place.

By default, floatmode="maxprec_equal" unless overridden.

Return Value

None, since this method just prints on the screen.


Basic usage

To show 3 fractional digits:

a = np.array([0.000005, 3.1416])
np.arraystring(a, precision=3)
'[5.000e-06 3.142e+00]'

To have unfixed precision, pass a None:

a = np.array([3.14, 3.1416])
np.arraystring(a, precision=None)
'[3.14 3.1416]'

Specifying max_line_width

By default, max_line_width=75, which means that each printed line can have at most 75 characters:

print(np.array([12, 34, 5]))
[12 34 5]

To print at most only 7 characters per line:

a = np.array([12, 34, 5])
np.arraystring(a, max_line_width=7)
'[12 34\n 5]'

This is what you see when you actually print the returned string:

print(np.arraystring(a, max_line_width=7))
[12 34

Specifying suppress_small

To show all decimal places for numbers smaller than 1e-4:

a = np.array([1e-5])
np.arraystring(a, suppress_small=True)

The default behavior of suppress=False gives us the following:

a = np.array([1e-5])

Specifying separator

To override the default delimiter of a single whitespace, pass in the separator parameter:

a = np.array([3,4,5])
np.arraystring(a, separator="A")

Specifying suppress

To show all decimal places for numbers smaller than 1e-4:

a = np.array([1e-5])
np.arraystring(a, suppress_small=True)

The default behaviour of suppress=False gives us the following:

a = np.array([1e-5])

Specifying prefix and suffix

The length of prefix and suffix is used to add padding to each line:

a = np.arange(5)
np.arraystring(a, max_line_width=15, prefix="BB", suffix="AAA")
'[0 1 2 3\n 4]'

Here, each line is formatted as follows:

prefix + array2string(a) + suffix

Each line can only hold a maximum of 15-len(prefix)-len(suffix)=10 characters. The actual content of prefix and suffix does not matter at all since they are not printed out - only their lengths matter.

Specifying formatter

To convert all boolean True to 1 and False to "-1".

mapping = {
"bool": lambda x: "1" if x else "-1"

a = np.array([True, False, True])
np.arraystring(a, formatter=mapping)
[1 -1 1]

Here, make sure you return a string in the mapping, otherwise an error will be thrown.

Specifying threshold

By default, threshold=1000, which means that arrays that have 1000 values or more will be summarised:

a = np.arange(1500)
[ 0 1 2 ... 1497 1498 1499]

This means that small arrays will not be summarised:

a = np.arange(7)
[0 1 2 3 4 5 6]

We can set a threshold so that even these small arrays will be summarised:

a = np.arange(7)
'[0 1 2 ... 4 5 6]'

Specifying edgeitems

By default, edgeitems=3, which means that when values are summarised, 3 values will be shown on the left, and 3 on the right:

a = np.arange(1500)
'[ 0 1 2 ... 1497 1498 1499]'

We can customise this by setting our own edgeitems:

a = np.arange(1500)
np.arraystring(a, edgeitems=4)
'[ 0 1 2 3 ... 1496 1497 1498 1499]'

Specifying sign

To show the + sign for positive numbers:

a = np.array([np.inf, 3.14, -2])
np.arraystring(a, sign="+")
'[ +inf +3.14 -2. ]'

To add a " " in front of positive numbers:

a = np.array([np.inf, 3.14, -2])
np.arraystring(a, sign=" ")
'[ inf 3.14 -2. ]'

It's hard to see here, but a single whitespace has been added.

Specifying floatmode


To obtain floats with the same decimal places (i.e. 8 by default):

a = np.array([5.05, 5.05001])
np.arraystring(a, floatmode="fixed")
'[5.05000000 5.05001000]'


To obtain floats with the minimum number of decimal places so as to uniquely identify the values:

a = np.array([5.05, 5.05001])
np.arraystring(a, floatmode="unique")
'[5.05 5.05001]'

Note that this ignores the precision parameter.


Same as unique, but the floats can have at most precision:

a = np.array([5.05, 5.052999])
np.arraystring(a, floatmode="maxprec", precision=4)
'[5.05 5.053]'


Same as maxprec, but the floats will all have the same precision:

a = np.array([5.05, 5.05001])
np.arraystring(a, floatmode="maxprec_equal")
'[5.05000 5.05001]'
Published by Isshin Inada
Edited by 0 others
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