functools.Placeholder

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By reading this article you will understand what functools.Placeholder is for and how to use it effectively.

Partial function application

The new Placeholder, added in Python 3.14, only makes sense in the context of functools.partial, so in order to understand Placeholder you will need to understand how functools.partial works and how to use it.

In a nutshell, partial allows you to perform partial function application, by “freezing” arguments to functions.

How to pass arguments to functools.partial

Up until Python 3.13, you could use partial to freeze arguments in two types of ways:

  1. you could pass positional arguments to partial, which would be passed in the same order to the function being used with partial; or
  2. you could pass keyword arguments to partial, which would be passed with the same name to the function being used with partial.

Using keyword arguments to skip the first argument

The method 2. is especially useful if you're trying to freeze an argument that is not the first one. For example, if you use the built-in help on the built-in int, you can see this signature:

int(x, base=10) -> integer

If you want to convert a binary string to an integer, you can set base=2:

print(int("101", 2))  # 5

Now, suppose you want to create a function from_binary by “freezing” the argument 2 in the built-in int. Writing

from_binary = partial(int, 2)

won't work, since in partial(int, 2), the value 2 is seen as the argument x from the signature above. However, you can pass the base as a keyword argument, skipping the first argument x from the signature of the built-in int:

from functools import partial

from_binary = partial(int, base=2)

print(from_binary("101"))  # 5

But this doesn't always work.

When keyword arguments don't work

Consider the following function that uses the string methods maketrans and translate to strip punctuation from a string:

import string

_table = str.maketrans("", "", string.punctuation)
def remove_punctuation(string):
    return string.translate(_table)

print(remove_punctuation("Hello, world!"))  # Hello world

The function remove_punctuation is a thin wrapper around the string method str.translate, which is the function doing all the work. In fact, if you look at str.translate as a function, you always pass _table as the second argument; what changes is the first argument:

print(str.translate("Hello, world!", _table))  # Hello world
print(str.translate("What?!", _table))  # What

This may lead you to wanting to use partial to freeze the value _table on the function str.translate, so you use the built-in help to check the signature of str.translate:

translate(self, table, /) unbound builtins.str method

You can see that the first argument is self, the string you are trying to translate, and then table is the translation table (that str.maketrans built magically for you). But you can also see the forward slash /, which means that self and table are positional-only arguments that cannot be passed in as keyword arguments!

As such, you cannot write the following code:

from functools import partial
import string

_table = str.maketrans("", "", string.punctuation)
remove_punctuation = partial(str.translate, table=_table)

print(remove_punctuation("Hello, world!"))
# TypeError: str.translate() takes no keyword arguments

But you also can't pass _table as a position argument to partial because the first argument must be the string to translate...

In a way, you want partial to capture the pattern str.translate(???, _table)... And that's what functools.Placeholder does.

Using functools.Placeholder to reserve a place for an argument

The new Placeholder can be used exactly for this: to reserve a place for a positional argument that will only be filled later. Using Python 3.14 (or later), you can write this code:

from functools import partial, Placeholder
import string

_table = str.maketrans("", "", string.punctuation)
remove_punctuation = partial(str.translate, Placeholder, _table)  # <--

print(remove_punctuation("Hello, world!"))  # Hello world

The key line is

remove_punctuation = partial(str.translate, Placeholder, _table)  # <--

When you call remove_punctuation(string), partial sees that you used a Placeholder earlier and it puts the string in that place, turning it into str.translate(string, _table).

Note that Placeholder is not a class you need to instantiate. Instead, it's a singleton (like None, for example) that you use directly whenever you need it.

Making Placeholder a bit more ergonomic

Since Placeholder is quite a mouthful – or a “keyboard-ful”, maybe –, you may want to import Placeholder with a shorter name. For example, importing Placeholder as _ leads to usages of Placeholder that look kind of neat:

from functools import partial, Placeholder as _
import string

_table = str.maketrans("", "", string.punctuation)
remove_punctuation = partial(str.translate, _, _table)  # <--

print(remove_punctuation("Hello, world!"))  # Hello world

Using a single underscore _ as shorthand for Placeholder is dangerous, though, because _ is typically used as a throwaway variable and you'll shadow Placeholder. A more viable alternative is _P, which is still pretty short.

Conclusion

From Python 3.14, you can use partial with more functions and in more scenarios, specifically with functions that accept some arguments as positional-only. In this article you saw an example of using Placeholder with the string method str.translate to create a function that removes punctuation from strings.

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