Weird behaviour initializing a numpy array of string data

The numpy string array is limited by its fixed length (length 1 by default). If you're unsure what length you'll need for your strings in advance, you can use dtype=object and get arbitrary length strings for your data elements:

my_array = numpy.empty([1, 2], dtype=object)

I understand there may be efficiency drawbacks to this approach, but I don't have a good reference to support that.

I got a "codec error" when I tried to use a non-ascii character with dtype="S10"

You also get an array with binary strings, which confused me.

I think it is better to use:

my_array = numpy.empty([1, 2], dtype="<U10")

Here 'U10' translates to "Unicode string of length 10; little endian format"

Numpy requires string arrays to have a fixed maximum length. When you create an empty array with dtype=str, it sets this maximum length to 1 by default. You can see if you do my_array.dtype; it will show "|S1", meaning "one-character string". Subsequent assignments into the array are truncated to fit this structure.

You can pass an explicit datatype with your maximum length by doing, e.g.:

my_array = numpy.empty([1, 2], dtype="S10")

The "S10" will create an array of length-10 strings. You have to decide how big will be big enough to hold all the data you want to hold.