functools.partial

This article teaches you how to use functools.partial, how it works, and when to use it, with clear examples.

functools.partial

What is functools.partial?

functools.partial is a tool from the standard module functools that allows you to curry positional and keyword arguments in functions. In a certain way, it's as if partial creates a specialised version of the function you pass it in, with certain arguments frozen.

For example, the built-in int converts objects to integers:

>>> int("14")
14
>>> int(14.5)
14

Maybe you didn't know that the built-in int also accepts a second argument that specifies the base from which the number must be converted:

>>> bin(99)  # 99 in binary is 1100011
'0b1100011'
>>> int("1100011", base=2)
99

>>> hex(99)  # 99 in hexadecimal is 63
'0x63'
>>> int("63", base=16)
99

By using functools.partial, you can create specialised versions of int where the parameter base was fixed to a certain base:

# partial lives inside the module functools:
>>> from functools import partial

>>> from_bin = partial(int, base=2)
>>> from_bin("1100011")
99

>>> from_hex = partial(int, base=16)
>>> from_hex("63")
99

The two examples above show that the first argument to partial is the function whose argument(s) we want to freeze. Then, we can provide as many positional or keyword arguments as needed. In our case, we just specified the parameter base as a keyword parameter. Here's how you could read both examples of partial:

• partial(int, base=2) - create a new version of int where the parameter base is always set to 2; and
• partial(int, base=16) – create a new version of int where the parameter base is always set to 16.

How to use functools.partial

The example above showed briefly how partial works and this section will go over the most important details. For demonstration purposes, let us create a function with a couple of parameters:

def foo(a, b, *, c, d=10):
print(a, b, c, d)

The function foo above has 4 parameters, of which c and d are keyword-only and d has a default value of 10.

functools.partial and positional arguments

partial can be given positional parameters, which will be passed in, in order, to the function that is the first argument to partial. When you call the new function, the other positional arguments you pass in are appended to the ones you already specified. So, bar = partial(foo, 1) is a function that has 3 parameters and that corresponds to foo(1, b, *, c, d=10). Similarly, baz = partial(foo, 1, 2) is a function that has 2 parameters and that corresponds to foo(1, 2, *, c, d=10).

>>> bar = partial(foo, 1)
>>> bar(20, c=30, d=40)
1 20 30 40

>>> baz = partial(foo, 1, 2)
>>> baz(c=30, d=40)
1 2 30 40

Notice that partial doesn't do any type of validation whatsoever regarding the number of arguments you pass in. This means that a call to partial might succeed but then produce a function that is unusable.

The example below shows this. The partial call below succeeds but then we're then unable to call too_many_args because foo, the underlying function, only accepts two positional arguments:

>>> too_many_args = partial(foo, 1, 2, 3)  # This works fine...

>>> too_many_args()  # ... even though too_many_args is unusable...
TypeError: foo() takes 2 positional arguments but 3 were given

>>> too_many_args(c=3, d=4)
TypeError: foo() takes 2 positional arguments but 3 positional arguments (and 2 keyword-only arguments) were given

functools.partial and keyword arguments

In the very first example we've already seen that partial accepts keyword arguments which are then passed into the underlying function. Keyword arguments set in partial can also be overridden:

>>> foo_no_d = partial(foo, d=999)

>>> foo_no_d(1, 2, c=3)  # d was set...
1 2 3 999

>>> foo_no_d(1, 2, c=3, d=4)  # ... but it can be overridden.
1 2 3 4

A very useful thing to note is that we can pass in arguments as keyword arguments to partial even if they're not necessarily keyword arguments in the original function. This means that we can use partial(foo, b=...) to freeze the value of the parameter b while leaving a unspecified, for example:

>>> foo_no_b = partial(foo, b=999)

>>> foo_no_b(1, c=3, d=4)
1 999 3 4

What we can't do is try to override the value of b by passing a second positional argument:

>>> foo_no_b(1, 2, c=3, d=4)
TypeError: foo() got multiple values for argument 'b'

Finally, bear in mind something that may or may not be obvious, which is that if a parameter is defined as positional-only, then you can't set it as a keyword with partial:

>>> def pos_only(a, /, b):  # The / says that a is positional-only.
...     print(a, b)
...
>>> pos_only(1, 2)
1 2
>>> pos_only(a=1, b=2)  # a can't be specified as a keyword value...
TypeError: pos_only() got some positional-only arguments passed as keyword arguments: 'a'

>>> f = partial(pos_only, a=1)
>>> f(b=2)
TypeError: pos_only() got some positional-only arguments passed as keyword arguments: 'a'

The partial object and its attributes

In the beginning of this article I wrote that “partial is a tool” instead of “partial is a function”, and I did that because partial is not a function. In fact, partial is a class:

>>> type(partial)
<class 'type'>

This means that using partial creates partial objects:

>>> from_bin = partial(int, base=2)
>>> from_bin
functools.partial(<class 'int'>, base=2)

This is not terribly important or worrying, although it does mean you get access to three read-only attributes:

Attribute name Description
func The original function passed into partial.
args The leftmost positional arguments for the function.
keywords The keyword arguments for the function.

Using the same foo as before, we can see all these three attributes in action:

>>> spam = partial(foo, 1, c=3, d=999)

>>> spam.func
<function foo at 0x1035a25e0>
>>> spam.func is foo
True

>>> spam.args
(1,)

>>> spam.keywords
{'c': 3, 'd': 999}

Knowing this, we can understand the simplified implementation of partial that the documentation shows:

def partial(func, /, *args, **keywords):
def newfunc(*fargs, **fkeywords):
newkeywords = {**keywords, **fkeywords}
return func(*args, *fargs, **newkeywords)
newfunc.func = func
newfunc.args = args
newfunc.keywords = keywords
return newfunc

The line newkeywords = {...} shows that keyword arguments can be overridden and the line return func(*args, *fargs, ...) shows that the positional arguments you pass into partial are used before the positional arguments passed in when you call the function.

functools.partial versus lambda

There are many practical cases in which using partial or lambda ends up being similar. For example, the from_bin and from_hex examples could've been lambda functions:

>>> from_bin = lambda x: int(x, base=2)
>>> from_bin("1100011")
99

>>> from_hex = lambda x: int(x, base=16)
>>> from_hex("63")
99

From the functional point of view, the two versions above are essentially the same as the two versions using partial and in some contexts deciding between a partial or a lambda may be a matter of personal preference. However, do keep in mind that partial provides much better introspection capabilities, whereas lambda is pretty opaque. Finally, most people consider that naming a lambda is an anti-pattern and something you shouldn't do.

partial is also much more convenient when freezing arguments in a function with a long signature because the lambda definition would have to recreate the whole signature. For example, if we only want to freeze the parameter a in foo, the version with lambda becomes more verbose:

from functools import partial
partial(foo, 1)

lambda b, *, c, d=10: foo(1, b, c=c, d=d)

The partial version is also more convenient to update.

When to use functools.partial

There are two occasions that are typically good use cases for partial, and that's when you want to create callbacks and when you want to create specialised versions of general functions.

Create callbacks

One example application for partial is to create callbacks in code that interacts with frameworks.

The function bt_draw below is a function from a tkinter calculator. Among other things, the function accepts a key that determines whether the button is a digit, an operator like + or -, or the key to clear the calculator. Then, the function bt_press is in charge of handling button presses.

To connect each button to the function bt_press, the first line of bt_draw uses command to specify which function must be called when the button is pressed. This function that will be called later is the callback. In our case, we say that the callback is the function bt_press. In order for bt_press to know what was the actual key that was pressed, when we specify the callback, we freeze the value of the key.

In the original calculator code, this was done with lambda:

def bt_draw(key, col, lin):
bt = tk.Button(window, text=key, command=lambda: bt_press(key))  # <--
bt.grid(column=col+1, row=lin+1)
return bt

def bt_press(key):
...

This could also be done with partial:

def bt_draw(key, col, lin):
bt = tk.Button(window, text=key, command=partial(bt_press, key))  # <--
...

Is partial a much better solution here? Not necessarily! lambda was perfectly fine in this case and in situations like this, where we are creating callbacks, I think that partial is better than lambda only when you need introspection or when the callbacks have long signatures.

Create specialised functions

The situation where partial really shines, in my opinion, is when you have a general function and you want to specialise it. Going back to the int example, notice how using partial to freeze the value of base allowed me to create two specialised versions of int:

1. one that converts binary strings into integers; and
2. one that converts hexadecimal strings into integers.

By creating these specialised versions, I created two functions that are simpler to use and are more well-defined. In particular, if the name is well chosen, the specialised versions become very convenient and may improve the readability of your code.

The benefits of using partial to create specialised functions increase with the complexity of the original function and/or the combination of arguments you are specifying. For example, if you specify base=0, int will interpret the string as an integer literal, which lets you convert things like 0b1100011 or 0x63 to integers, and int takes care of figuring out the base:

>>> int("0b1100011", base=0)
99
>>> int("0x63", base=0)
99

By using partial with base=0, we can create a specialised function with a better name that makes it easier for others to understand what is happening if they don't know what base=0 does yet:

>>> interpret_int_literal = partial(int, base=0)
>>> interpret_int_literal("0b1100011")
99
>>> interpret_int_literal("0x63")
99

The fact that we use the name interpret_int_literal is a great hint as to what int(..., base=0) does, which is helpful to those who are reading the code and never encountered the parameter base=0.

Another example I enjoy a lot is that of using max/min/sorted with the key parameter set to len. When setting key to the built-in len, these functions get a new meaning:

partial object Meaning
partial(max, key=len) Find longest item
partial(min, key=len) Find shortest item
partial(sorted, key=len) Sort by length

Here's an example application:

>>> longest = partial(max, key=len)
>>> longest("The quick brown fox jumps over the lazy dog".split())
'quick'

Table of partial examples

Just for the fun of it, the table below includes many examples of using partial that take a general function and create a more specific function with a more specific meaning. The main purpose of this table is to show the relationships between some built-ins and some concepts. Thus, not all examples of partial here would be suitable for use in production code.

As an exercise, use each one of these partial objects.

Original partial application Meaning
int partial(int, base=2) Read binary numbers
int partial(int, base=16) Read hexadecimal numbers
max partial(max, key=len) Find longest item
min partial(min, key=len) Find shortest item
sorted partial(sorted, key=len) Sort by length
max partial(max, default=float("-inf")) Mathematically correct max
min partial(min, default=float("inf")) Mathematically correct min
round partial(round, ndigits=2) Round to 2 decimal places
round partial(round, ndigits=-3) Round to nearest thousand
reduce partial(reduce, operator.add) sum built-in1
reduce partial(reduce, operator.mul) prod from the module math2
reduce partial(reduce, opertator.and_) all built-in3
reduce partial(reduce, operator.or_) any built-in4
range partial(range, 1) Natural counting
enumerate partial(enumerate, start=1) Natural enumeration

If you come up with other interesting examples of partial, feel free to comment them below or to email me and I might add them here!

1. The partial application is not equivalent to sum because sum([]) gives 0 and the partial application will error.

2. The partial application is not equivalent to prod because prod([]) gives 1 and the partial application will error.

3. The partial application is not equivalent to all because all([]) gives True and the partial application will error. Furthermore, all will short-circuit and the partial application won't.

4. The partial application is not equivalent to any because any([]) gives False and the partial application will error. Furthermore, any will short-circuit and the partial application won't.

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