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Python 3.4 (3.3 is also afflicted:
>>> import struct
>>> struct.Struct(3)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: Struct() argument 1 must be a bytes object, not int
>>> struct.Struct('b')
<Struct object at 0x7fec04763180>
Python 2.7:
>>> import struct
>>> struct.Struct(3)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: Struct() argument 1 must be string, not int
>>> struct.Struct(u'b')
<Struct object at 0x17993e8>
Here is the patch to better error message for Python 3.4 and 3.3.

And here is the patch to better error message in Python 2.7.

I think that error message in 2.7 is correct. "String" means both str and unicode.
As for 3.x, agree, it should be corrected. But I prefer "str or bytes" or "string or bytes object".

Here is the patch to address Serhiy's request.
Hmmm, if string means both string and unicode in Python 2.7, should we fix these behaviours?
>>> import _csv
>>> _csv.register_dialect(2)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: dialect name must be a string or unicode
>>> ' cute cat '.strip(3)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: strip arg must be None, str or unicode
>>> import sqlite3
>>> conn = sqlite3.connect(':memory:')
>>> c = conn.cursor()
>>> c.execute(3)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: operation parameter must be str or unicode

Closely related:
* Issue 16349: document byte string format argument support
* Issue 21071: should the Struct.format property be bytes or text?

For Python 2.7, this change doesn’t seem important enough for a bug fix.