Is it possible to have all "git diff" commands use the "Python diff", in all git projects?

Quoting from gitattributes(5):

Attributes that should affect all repositories for a single user should be placed in a file specified by the core.attributesfile configuration option (see git-config(1)). Its default value is $XDG_CONFIG_HOME/git/attributes. If $XDG_CONFIG_HOME is either not set or empty, $HOME/.config/git/attributes is used instead. Attributes for all users on a system should be placed in the $(prefix)/etc/gitattributes file.

TL;DR: echo '*.py diff=python' >> "${XDG_CONFIG_HOME:-$HOME/.config}"/git/attributes

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Update, 7 years later

Ok, it's not necessary to configure diff=python for *.py files — it's the default since long ago.

But the general point remains: anything you can set up in local (per-repository) .gitattributes, you can also make global (per-machine).

There're many good examples in man 5 gitattributes itself, so please go RTFM.

Let's do just one custom setup: --word-diff for all Markdown files (kudos to @RayLuo for suggesting this in comments).

First, we add an external diff driver:

git config --global diff.stackoverflow-word-diff.command ~/.local/bin/stackoverflow-word-diff

The API is such that we must make a standalone wrapper executable.

cat > ~/.local/bin/stackoverflow-word-diff << 'EOF'
#!/bin/bash -eu

#-- uncomment for debug:
#echo >&2 "$(basename $0) args: $@"; set -x

FILENAME="$1"
OLDFILE="$2"
OLDHASH="$3"
OLDMODE="$4"
NEWFILE="$5"
NEWHASH="$6"
NEWMODE="$7"

git diff --no-ext-diff --word-diff "$OLDFILE" "$NEWFILE" || exit 0

#-- from https://stackoverflow.com/a/18948381/531179
#-- see `man 1 git` /EXTERNAL_DIFF, or https://www.git-scm.com/docs/git
EOF
chmod +x ~/.local/bin/stackoverflow-word-diff

Finally, we tie that in to *.md, *.markdown via global gitattributes:

mkdir -vp "${XDG_CONFIG_HOME:-$HOME/.config}"/git

{ echo '*.md diff=stackoverflow-word-diff'; \
  echo '*.markdown diff=stackoverflow-word-diff; \
} \
    >> "${XDG_CONFIG_HOME:-$HOME/.config}"/git/attributes

And that's all folks! Test it.

To tell git to use ~/.gitattributes you need to put this in ~/.gitconfig:

[core]
  attributesfile = ~/.gitattributes