brew install ffmpeg installs python 3.9

Question

For some reason brew install ffmpeg downloads python 3.9 as dependence

==> Installing dependencies for ffmpeg: gnutls, python@3.9, glib, cairo, gobject-introspection, harfbuzz, libass and libvpx

As I can see here https://formulae.brew.sh/formula/ffmpeg there is no dependence on python 3.9

How can I install ffmpeg for my current python Python 3.7.7?

Update:

Maybe something is broken in my python installation?

brew info python

python@3.9: stable 3.9.0 (bottled)
Interpreted, interactive, object-oriented programming language
https://www.python.org/
/usr/local/Cellar/python/3.7.7 (4,165 files, 64.0MB) *
  Poured from bottle on 2020-04-03 at 20:11:58
From: https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/python@3.9.rb
License: Python-2.0
==> Dependencies
Build: pkg-config ✔
Required: gdbm ✔, openssl@1.1 ✔, readline ✔, sqlite ✔, xz ✔
==> Caveats
Python has been installed as
  /usr/local/bin/python3

Unversioned symlinks `python`, `python-config`, `pip` etc. pointing to
`python3`, `python3-config`, `pip3` etc., respectively, have been installed into
  /usr/local/opt/python@3.9/libexec/bin

You can install Python packages with
  pip3 install <package>
They will install into the site-package directory
  /usr/local/lib/python3.9/site-packages

See: https://docs.brew.sh/Homebrew-and-Python
==> Analytics
install: 588,344 (30 days), 666,372 (90 days), 666,373 (365 days)
install-on-request: 104,126 (30 days), 105,703 (90 days), 105,705 (365 days)
build-error: 0 (30 days)

brew info shows python@3.9 but actually it's 3.7.7:

/usr/local/bin/python3 -V
Python 3.7.7

Answer 1

Using brew deps --tree ffmpeg shows the dependancy tree;

Here you can find that the package glib requires python@3.9. There are some bug reports mentioning this.

You could install ffmpeg without any deps after you manually installed any required deps;

brew install --ignore-dependencies ffmpeg

ffmpeg
├── ...
├── libass
│   ├── freetype
│   │   └── libpng
│   ├── fribidi
│   └── harfbuzz
│       ├── cairo
│       │   ├── fontconfig
│       │   │   └── freetype
│       │   │       └── libpng
│       │   ├── freetype
│       │   │   └── libpng
│       │   ├── glib
│       │   │   ├── gettext
│       │   │   ├── libffi
│       │   │   ├── pcre
│       │   │   └── python@3.9
│       │   │       ├── gdbm
│       │   │       ├── openssl@1.1
│       │   │       ├── readline
│       │   │       ├── sqlite
│       │   │       │   └── readline
│       │   │       └── xz
│       │   ├── libpng
│       │   ├── lzo
│       │   └── pixman
│       ├── freetype
│       │   └── libpng
│       ├── glib
│       │   ├── gettext
│       │   ├── libffi
│       │   ├── pcre
│       │   └── python@3.9
│       │       ├── gdbm
│       │       ├── openssl@1.1
│       │       ├── readline
│       │       ├── sqlite
│       │       │   └── readline
│       │       └── xz
│       ├── gobject-introspection
│       │   ├── cairo
│       │   │   ├── fontconfig
│       │   │   │   └── freetype
│       │   │   │       └── libpng
│       │   │   ├── freetype
│       │   │   │   └── libpng
│       │   │   ├── glib
│       │   │   │   ├── gettext
│       │   │   │   ├── libffi
│       │   │   │   ├── pcre
│       │   │   │   └── python@3.9
│       │   │   │       ├── gdbm
│       │   │   │       ├── openssl@1.1
│       │   │   │       ├── readline
│       │   │   │       ├── sqlite
│       │   │   │       │   └── readline
│       │   │   │       └── xz
│       │   │   ├── libpng
│       │   │   ├── lzo
│       │   │   └── pixman
│       │   ├── glib
│       │   │   ├── gettext
│       │   │   ├── libffi
│       │   │   ├── pcre
│       │   │   └── python@3.9
│       │   │       ├── gdbm
│       │   │       ├── openssl@1.1
│       │   │       ├── readline
│       │   │       ├── sqlite
│       │   │       │   └── readline
│       │   │       └── xz
│       │   ├── libffi
│       │   ├── pkg-config
│       │   └── python@3.9
│       │       ├── gdbm
│       │       ├── openssl@1.1
│       │       ├── readline
│       │       ├── sqlite
│       │       │   └── readline
│       │       └── xz
│       ├── graphite2
│       └── icu4c
├── ...