My code:

class Git:

$\_\_$slots$\_\_=["$current$\_$branch", "start", "selected$\_$commit", "visited$\_$branches", "branches"$]$

def $\_\_$init$\_\_($self$)$:

# Reference to the original$/$main branch

self.start $=$ GitBranch$()$

self.current$\_$branch $=$ self.start

self.selected$\_$commit: Node $=$ self.start.get$\_$last$\_$commit$()$

self.visited$\_$branches $=$ set$()$

# Maintain a dictionary of all branches

self.branches $=\{"$main$"$: self.start$\}$

def get$\_$current$\_$commit$($self$)->$ Optional$[$str$]$:

$"""$

Return the value stored in the currently selected commit.

:return: Value of the current working commit.

$"""$

if self.selected$\_$commit:

return self.selected$\_$commit.value

return None

def get$\_$current$\_$branch$\_$name$($self$)->$ Optional$[$str$]$:

$"""$

Return the name of the current working$/$active branch.

:return: Name of the current working branch.

$"""$

return self.current$\_$branch.name

def commit$($self$,$ message: str$)->$ None:

$"""$

Commit to the timeline if it is the last element in the commit.

If the current working commit is not the last commit, raise an exception.

:param message: Message to be added to the commit.

$"""$

if self.selected$\_$commit $!=$ self.current$\_$branch.get$\_$last$\_$commit$()$:

raise Exception$("$Cannot commit: not at the last commit in the branch."$)$

self.selected$\_$commit $=$ self.current$\_$branch.push$\_$commit$($message$)$

def backwards$($self$)->$ None:

$"""$

Moves the reference of the current working commit back one commit.

If already at the first commit of the branch, switch back to the parent branch if available.

$"""$

if self.selected$\_$commit:

print$($

f$"$Before moving backwards, current commit: $\{$self$.$get$\_$current$\_$commit$()\}$ on branch $\{$self$.$get$\_$current$\_$branch$\_$name$()\}")$

if self.selected$\_$commit and self.selected$\_$commit.prev:

# Move to the previous commit in the current branch

self.selected$\_$commit $=$ self.selected$\_$commit.prev

print$($f$"$Moved Back to Commit: $\{$self$.$selected$\_$commit.value$\}")$

# If we are at the branching point $($the parent node$),$ switch to the parent branch

elif self.selected$\_$commit $==$ self.current$\_$branch.parent$\_$node:

# Switch back to the main branch

self.current$\_$branch $=$ self.start # Main branch starts here

self.selected$\_$commit $=$ self.current$\_$branch.get$\_$last$\_$commit$()$ # Set the commit to the last commit on the main branch

print$($f$"$Switched back to 'main' branch, current commit: $\{$self$.$get$\_$current$\_$commit$()\}")$

else:

print$($f$"$Already at the first commit of the branch: $\{$self$.$get$\_$current$\_$commit$()\}")$

def forward$($self$)->$ None:

$"""$

Move the reference of the current working commit forward one commit within the current branch.

If already in the last commit of the branch, do not jump to a child branch automatically.

$"""$

if self.selected$\_$commit and self.selected$\_$commit.next:

# Move to the next commit in the current branch

self.selected$\_$commit $=$ self.selected$\_$commit.next

print$($f$"$Moved Forward to Commit: $\{$self$.$selected$\_$commit.value$\}")$

else:

print$($f$"$Already at the last commit of the branch: $\{$self$.$get$\_$current$\_$commit$()\}")$

# DO NOT MODIFY BELOW #

# The following methods are already implemented for you to $(1)$ better understand how Git works,

# $(2)$ better understand tests.py as they are called in tests.py$.$

def checkout$\_$commit$($self$,$ message$)->$ None:

$"""$

Check out any commit in the tree, moving the current selected branch to that commit's branch.

If the commit is found, change the current selected branch to be the parent branch of the commit.

If no such commit exists, raise an exception.

:param message: Commit message to look for.

$"""$

existing$\_$commit $=$ self.find$\_$commit$($self$.$start, message$)$

if existing$\_$commit is not None:

self.current$\_$branch $=$ existing$\_$commit$[0]$

self.selected$\_$commit $=$ existing$\_$commit$[1]$

return

raise Exception$("$Commit is not existent"$)$

def checkout$\_$branch$($self$,$ name$)->$ None:

$"""$

Check out a tree branch, and move the working commit to the last commit on the branch.

If the branch with the given name already exist, change the current branch to be that one, and change the current

commit to be the last commit on the branch. If branch does not exist and current working commit does not have a

branch, then create a branch from that commit.

:param name: The branch name to look for.

:return: None.

$"""$

existing$\_$branch $=$ self.find$\_$branch$($self$.$start, name$)$