Pythonic way to verify parameter is a sequence but not string

Question

I have a function that gets a list of DB tables as parameter, and returns a command string to be executed on these tables, e.g.:

pg_dump( file='/tmp/dump.sql',
         tables=('stack', 'overflow'),
         port=5434
         name=europe)

Should return something like:

pg_dump -t stack -t overflow -f /tmp/dump.sql -p 5434 europe

This is done using tables_string='-t '+' -t '.join(tables).

The fun begins when the function is called with: tables=('stackoverflow') (a string) instead of tables=('stackoverflow',) (a tuple), which yields:

pg_dump -t s -t t -t a -t c -t k -t o -t v -t e -t r -t f -t l -t o -t w
        -f /tmp/dump.sql -p 5434 europe

Because the string itself is being iterated.

This SO question suggests using asserts on the type, but I'm not sure it's Pythonic enough because it breaks the duck-type convention.

Any insights?

Adam

Answer 1

Asserting the type seems appropriate in this case - handling a common misuse that seems legal because of duck typing.

Another way to handle this common case would be to test for string and handle it correctly as a special case.

Finally, you could encourage passing the table names as positional parameters which would make this scenario less likely:

def pg_dump(*tables, **kwargs):
  file = kwargs['file']
  port = kwargs['port']
  name = kwargs['name']
  ...

pg_dump('stack', 'overflow', file='/tmp/dump.sql', port=5434, name='europe')

Answer 2

You can use ABCs to assert that an object is iterable but not a string:

from types import StringType
from collections import Iterable
assert isinstance(x, Iterable) and not isinstance(x, StringType)

Answer 3

A common Python idiom to detect whether an argument is a sequence (a list or tuple) or a string is to check whether it has the __iter__ attribute:

def func(arg):
    if hasattr(arg, '__iter__'):
        print repr(arg), 'has __iter__ attribute'
    else:
        print repr(arg), 'has no __iter__ attribute'

func('abc')
# 'abc' has no __iter__

func(('abc'))
# 'abc' has no __iter__

func(('abc',))
# ('abc',) has __iter__

When it's not a sequence, it's also common to change it into one to simplify the rest of the code (which only has to deal with one kind of thing). In the sample it could have been done with a simple arg = [arg].

Answer 4

Can you not use a list rather than a tuple?

pg_dump( file='/tmp/dump.sql',
         tables=['stack', 'overflow'],
         port=5434,
         name='europe')