What's the difference between dict() and {}?

Question

So let's say I wanna make a dictionary. We'll call it d. But there are multiple ways to initialize a dictionary in Python! For example, I could do this:

d = {'hash': 'bang', 'slash': 'dot'}

Or I could do this:

d = dict(hash='bang', slash='dot')

Or this, curiously:

d = dict({'hash': 'bang', 'slash': 'dot'})

Or this:

d = dict([['hash', 'bang'], ['slash', 'dot']])

And a whole other multitude of ways with the dict() function. So obviously one of the things dict() provides is flexibility in syntax and initialization. But that's not what I'm asking about.

Say I were to make d just an empty dictionary. What goes on behind the scenes of the Python interpreter when I do d = {} versus d = dict()? Is it simply two ways to do the same thing? Does using {} have the additional call of dict()? Does one have (even negligible) more overhead than the other? While the question is really completely unimportant, it's a curiosity I would love to have answered.

Answer 1

>>> def f():
...     return {'a' : 1, 'b' : 2}
... 
>>> def g():
...     return dict(a=1, b=2)
... 
>>> g()
{'a': 1, 'b': 2}
>>> f()
{'a': 1, 'b': 2}
>>> import dis
>>> dis.dis(f)
  2           0 BUILD_MAP                0
              3 DUP_TOP             
              4 LOAD_CONST               1 ('a')
              7 LOAD_CONST               2 (1)
             10 ROT_THREE           
             11 STORE_SUBSCR        
             12 DUP_TOP             
             13 LOAD_CONST               3 ('b')
             16 LOAD_CONST               4 (2)
             19 ROT_THREE           
             20 STORE_SUBSCR        
             21 RETURN_VALUE        
>>> dis.dis(g)
  2           0 LOAD_GLOBAL              0 (dict)
              3 LOAD_CONST               1 ('a')
              6 LOAD_CONST               2 (1)
              9 LOAD_CONST               3 ('b')
             12 LOAD_CONST               4 (2)
             15 CALL_FUNCTION          512
             18 RETURN_VALUE

dict() is apparently some C built-in. A really smart or dedicated person (not me) could look at the interpreter source and tell you more. I just wanted to show off dis.dis. :)

Answer 2

As far as performance goes:

>>> from timeit import timeit
>>> timeit("a = {'a': 1, 'b': 2}")
0.424...
>>> timeit("a = dict(a = 1, b = 2)")
0.889...

Answer 3

Basically, {} is syntax and is handled on a language and bytecode level. dict() is just another builtin with a more flexible initialization syntax. Note that dict() was only added in the middle of 2.x series.

Answer 4

Update: thanks for the responses. Removed speculation about copy-on-write.

One other difference between {} and dict is that dict always allocates a new dictionary (even if the contents are static) whereas {} doesn't always do so (see mgood's answer for when and why):

def dict1():
    return {'a':'b'}

def dict2():
    return dict(a='b')

print id(dict1()), id(dict1())
print id(dict2()), id(dict2())

produces:

$ ./mumble.py
11642752 11642752
11867168 11867456

I'm not suggesting you try to take advantage of this or not, it depends on the particular situation, just pointing it out. (It's also probably evident from the disassembly if you understand the opcodes).

Answer 5

@Jacob: There is a difference in how the objects are allocated, but they are not copy-on-write. Python allocates a fixed-size "free list" where it can quickly allocate dictionary objects (until it fills). Dictionaries allocated via the {} syntax (or a C call to PyDict_New) can come from this free list. When the dictionary is no longer referenced it gets returned to the free list and that memory block can be reused (though the fields are reset first).

This first dictionary gets immediately returned to the free list, and the next will reuse its memory space:

>>> id({})
340160
>>> id({1: 2})
340160

If you keep a reference, the next dictionary will come from the next free slot:

>>> x = {}
>>> id(x)
340160
>>> id({})
340016

But we can delete the reference to that dictionary and free its slot again:

>>> del x
>>> id({})
340160

Since the {} syntax is handled in byte-code it can use this optimization mentioned above. On the other hand dict() is handled like a regular class constructor and Python uses the generic memory allocator, which does not follow an easily predictable pattern like the free list above.

Also, looking at compile.c from Python 2.6, with the {} syntax it seems to pre-size the hashtable based on the number of items it's storing which is known at parse time.