Truth value of a string in python

if <boolean> :
   # do this

boolean has to be either True or False.

then why

if "poi":
   print "yes"

output: yes

i didn't get why yes is printing , since "poi" is nether True or False.

Python will do its best to evaluate the "truthiness" of an expression when a boolean value is needed from that expression.

The rule for strings is that an empty string is considered False, a non-empty string is considered True. The same rule is imposed on other containers, so an empty dictionary or list is considered False, a dictionary or list with one or more entries is considered True.

The None object is also considered false.

A numerical value of 0 is considered false (although a string value of '0' is considered true).

All other expressions are considered True.

Details (including how user-defined types can specify truthiness) can be found here: http://docs.python.org/release/2.5.2/lib/truth.html.

In python, any string except an empty string defaults to True

ie,

if "MyString":
    # this will print foo
    print("foo")

if "":
    # this will NOT print foo
    print("foo")

What is happening here is Python' supplement of implicit bool() constructor after the if, Because anything followed by if should be resolved to be boolean. In this context your code is equivalent to

if bool("hello"):
   print "yes"

According to Python bool(x) constructor accepts anything and decides the truthiness based on below cases

• If x is integer, Only 0 is False everything else is True
• If x is float, Only 0.0 is False everything else is True`
• If x is list, Only [] is False everything else is True
• If x is set/dict, Only {} is False everything else is True
• If x is tuple, Only () is False everything else is True
• If x is string, Only “" is False everything else is True. Be aware that bool(“False”) will return to True

Here is the log for the cases I listed above

Python 3.4.3 (default, Feb 25 2015, 21:28:45) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.56)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> bool(0)
False
>>> bool(1)
True
>>> bool(-1)
True
>>> bool(0.0)
False
>>> bool(0.02)
True
>>> bool(-0.10)
True
>>> bool([])
False
>>> bool([1,2])
True
>>> bool(())
False
>>> bool(("Hello","World"))
True
>>> bool({})
False
>>> bool({1,2,3})
True
>>> bool({1:"One", 2:"Two"})
True
>>> bool("")
False
>>> bool("Hello")
True
>>> bool("False")
True