4 Truth and Consequences

There are no explicit boolean types in Python. Any variable or expression can be tested for truth in an if statement. The rules for whether something is considered true or false are:

None is false.
Numeric types:
- Nonzero values are true.
- Zero values (0, 0L, 0.0, complex 0+0j) are false.
Strings, lists, tuples, dictionaries:
- Empty values are false, so "", [], (), and {} are all false.
- Non-empty values are true (even if filled with zeros or None's, for example, [None] is true. "" is false, but the string "false" is true).
Other objects:
- All other objects are considered true (unless the programmer has done some fancy stuff to make them test as false). For example, after fh = open('filename'), the filehandle fh is true (even if the file is closed with fh.close()).

One cannot (mistakenly or deliberately) put an assignment statement inside a conditional as one can in C. For example the following is not legal Python:

if x = 0: print "did I really mean to write a never used print statement?"

But this is legal:

if x == 0: print "much better"

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Documented updated on 2004 Jun 1