What causes "binary tree cannot be empty" in TheAlgorithms/Python?

Check the dict before calling: `if not tree: ...` and handle the empty case explicitly

TheAlgorithms/Python · error · ValueError

binary tree cannot be empty

Error message

binary tree cannot be empty

What it means

Raised by binary_tree_mirror(binary_tree, root) when the binary_tree adjacency dict is falsy ({} or None). The function mirrors a dict-of-lists tree representation; an empty dict has no root to start the recursive swap, so it refuses with ValueError before doing work. Note the function also copies the input (dict(binary_tree)) — it never mutates the caller's tree.

Source

Thrown at data_structures/binary_tree/binary_tree_mirror.py:32


def binary_tree_mirror(binary_tree: dict, root: int = 1) -> dict:
    """
    >>> binary_tree_mirror({ 1: [2,3], 2: [4,5], 3: [6,7], 7: [8,9]}, 1)
    {1: [3, 2], 2: [5, 4], 3: [7, 6], 7: [9, 8]}
    >>> binary_tree_mirror({ 1: [2,3], 2: [4,5], 3: [6,7], 4: [10,11]}, 1)
    {1: [3, 2], 2: [5, 4], 3: [7, 6], 4: [11, 10]}
    >>> binary_tree_mirror({ 1: [2,3], 2: [4,5], 3: [6,7], 4: [10,11]}, 5)
    Traceback (most recent call last):
        ...
    ValueError: root 5 is not present in the binary_tree
    >>> binary_tree_mirror({}, 5)
    Traceback (most recent call last):
        ...
    ValueError: binary tree cannot be empty
    """
    if not binary_tree:
        raise ValueError("binary tree cannot be empty")
    if root not in binary_tree:
        msg = f"root {root} is not present in the binary_tree"
        raise ValueError(msg)
    binary_tree_mirror_dictionary = dict(binary_tree)
    binary_tree_mirror_dict(binary_tree_mirror_dictionary, root)
    return binary_tree_mirror_dictionary


if __name__ == "__main__":
    binary_tree = {1: [2, 3], 2: [4, 5], 3: [6, 7], 7: [8, 9]}
    print(f"Binary tree: {binary_tree}")
    binary_tree_mirror_dictionary = binary_tree_mirror(binary_tree, 5)
    print(f"Binary tree mirror: {binary_tree_mirror_dictionary}")

View on GitHub (pinned to f5988cc097)

Solutions

Check the dict before calling: `if not tree: ...` and handle the empty case explicitly
Fix the upstream builder so an empty tree is not silently produced (log when zero nodes are parsed)
Wrap the call in try/except ValueError if the empty case is legitimately possible in your flow

Example fix

# before
mirrored = binary_tree_mirror(adj, root)  # adj == {}

# after
if adj:
    mirrored = binary_tree_mirror(adj, root)
else:
    mirrored = {}

Defensive patterns

Strategy: validation

Validate before calling

def mirror_or_none(tree: dict) -> dict | None:
    if not tree:
        return None
    return binary_tree_mirror(tree, next(iter(tree)))

Type guard

def is_nonempty_tree(tree: object) -> bool:
    return isinstance(tree, dict) and len(tree) > 0 and all(isinstance(v, list) and len(v) == 2 for v in tree.values())

Try / catch

try:
    mirrored = binary_tree_mirror(tree, root)
except ValueError as e:
    if "cannot be empty" not in str(e):
        raise
    mirrored = {}

Prevention

Never build an adjacency dict without asserting at least one node was parsed
Log when a tree builder returns zero entries
Treat an empty tree as a valid edge case at the caller, not inside the library

When it happens

Trigger: binary_tree_mirror({}, 1) with an empty dict; passing a tree dict that a previous step built as {} because the parser/source produced no edges.

Common situations: Building the adjacency dict from user input or a file that yielded no nodes; upstream filtering that removed all entries; default-initialized dict never populated.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/cb86a2e4a13d6fb9. Report an issue: GitHub.