Starting with Python 3.6, you can:

>>> value = 42
>>> padding = 6
>>> f"{value:#0{padding}x}"
'0x002a'

Note the padding includes the 0x. If you don't want that you can do

>>> f"0x{value:0{padding}x}"
'0x00002a'

for older python versions use the .format() string method:

>>> "{0:#0{1}x}".format(42,6)
'0x002a'

Explanation:

{   # Format identifier
0:  # first parameter
#   # use "0x" prefix
0   # fill with zeroes
{1} # to a length of n characters (including 0x), defined by the second parameter
x   # hexadecimal number, using lowercase letters for a-f
}   # End of format identifier

If you want the letter hex digits uppercase but the prefix with a lowercase 'x', you'll need a slight workaround:

>>> '0x{0:0{1}X}'.format(42,4)
'0x002A'

This is happening because hex() will not include any leading zeros, for example:

>>> hex(15)[2:]
'f'

To make sure you always get two characters, you can use str.zfill() to add a leading zero when necessary:

>>> hex(15)[2:].zfill(2)
'0f'

Here is what it would look like in your code:

fc = '0x'
for i in b[0x15c:0x15f]:
    fc += hex(ord(i))[2:].zfill(2)

Perhaps you're looking for the .zfill method on strings. From the docs:

Help on built-in function zfill:

zfill(...)
    S.zfill(width) -> string

    Pad a numeric string S with zeros on the left, to fill a field
    of the specified width.  The string S is never truncated.

Your code can be written as:

def padhexa(s):
    return '0x' + s[2:].zfill(8)

assert '0x00000123' == padhexa('0x123')
assert '0x00ABCD12' == padhexa('0xABCD12')
assert '0x12345678' == padhexa('0x12345678')

(Recommended)

Python 3 f-strings: Answered by @GringoSuave

>>> i = 3735928559
>>> f'{i:x}'
'deadbeef'

Alternatives:

format builtin function (good for single values only)

>>> format(3735928559, 'x')
'deadbeef'

And sometimes we still may need to use str.format formatting in certain situations @Eumiro

(Though I would still recommend f-strings in most situations)

>>> '{:x}'.format(3735928559)
'deadbeef'

(Legacy) f-strings should solve all of your needs, but printf-style formatting is what we used to do @msvalkon

>>> '%x' % 3735928559
'deadbeef'

Without string formatting @jsbueno

>>> i = 3735928559
>>> i.to_bytes(4, "big").hex()
'deadbeef'

Hacky Answers (avoid)

hex(i)[2:] @GuillaumeLemaître

>>> i = 3735928559
>>> hex(i)[2:]
'deadbeef'

This relies on string slicing instead of using a function / method made specifically for formatting as hex. This is why it may give unexpected output for negative numbers:

>>> i = -3735928559
>>> hex(i)[2:]
'xdeadbeef'
>>> f'{i:x}'
'-deadbeef'