invocation-tree 0.0.14

Generate an invocation tree of functions calls.

Installation

Install (or upgrade) invocation_tree using pip:

pip install --upgrade invocation_tree

Additionally Graphviz needs to be installed.

Invocation Tree

The invocation_tree package is designed to help with program understanding and debugging by visualizing the tree of function invocations that occur during program execution. Here’s a simple example of how it works, we start with a = 1 and compute:

    (a - 3 + 9) * 6

import invocation_tree as ivt

def main():
    a = 1
    a = expression(a)
    return multiply(a, 6)
    
def expression(a):
    a = subtract(a, 3)
    return add(a, 9)
    
def subtract(a, b):
    return a - b

def add(a, b):
    return a + b

def multiply(a, b):
    return a * b

tree = ivt.blocking()
print( tree(main) ) # show invocation tree starting at main

Running the program and pressing <Enter> a number of times results in:

42

Each node in the tree represents a function call, and the node's color indicates its state:

• White: The function is currently being executed (it is at the top of the call stack).
• Green: The function is paused and will resume execution later (it is lower down on the call stack).
• Red: The function has completed execution and returned (it has been removed from the call stack).

For every function, the package displays its local variables and return value. Changes to these values over time are highlighted using bold text and gray shading to make them easy to track.

Chapters

Comprehensions

Debugger

Recursion

Configuration

Troubleshooting

Author

Bas Terwijn

Inspiration

Inspired by rcviz.

Supported by

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Comprehensions

In this more interesting example we compute which students pass a course by using list and dictionary comprehensions.

import invocation_tree as ivt
from decimal import Decimal, ROUND_HALF_UP

def main():
    students = {'Ann':[7.5, 8.0], 
                'Bob':[4.5, 6.0], 
                'Coy':[7.5, 6.0]}
    averages = {student:compute_average(grades)
                for student, grades in students.items()}
    passing = passing_students(averages)
    print(passing)

def compute_average(grades):
    average = sum(grades)/len(grades)
    return half_up_round(average, 1)
    
def half_up_round(value, digits=0):
    """ High-precision half-up rounding of 'value' to a specified number of 'digits'. """
    return float(Decimal(str(value)).quantize(Decimal(f"1e-{digits}"),
                                              rounding=ROUND_HALF_UP))

def passing_students(averages):
    return [student 
        for student, average in averages.items() 
        if average >= 5.5]

if __name__ == '__main__':
    tree = ivt.blocking()
    tree(main)

['Ann', 'Coy']

Blocking

The program blocks execution at every function call and return statement, printing the current location in the source code. Press the <Enter> key to continue execution. To block at every line of the program (like in a debugger tool) and only where a change of value occured, use instead:

    tree = ivt.blocking_each_change()

Debugger

To visualize the invocation tree in a debugger tool, such as the integrated debugger in Visual Studio Code, use instead:

    tree = ivt.debugger()

and open the 'tree.pdf' file manually.

Recursion

An invocation tree is particularly helpful to better understand recursion. A simple factorial() example:

import invocation_tree as ivt

def factorial(n):
    if n <= 1:
        return 1
    prev_result = factorial(n - 1)
    return n * prev_result

tree = ivt.blocking()
print( tree(factorial, 4) ) # show invocation tree of calling factorial(4)

24

This permutations() example shows the depth-first nature of recursive execution:

import invocation_tree as ivt

def permutations(elements, perm, n):
    if n==0:
        return [perm]
    all_perms = []
    for element in elements:
        all_perms.extend(permutations(elements, perm + element, n-1))
    return all_perms

tree = ivt.blocking()
result = tree(permutations, ['L','R'], '', 2)
print(result) # all permutations of going Left or Right of length 2

['LL', 'LR', 'RL', 'RR']

Hide Variables

In an educational context it can be useful to hide certian variables to avoid unnecessary complexity. This can for example be done with:

tree = ivt.blocking()
tree.hide.add('permutations.elements')
tree.hide.add('permutations.element')
tree.hide.add('permutations.all_perms')

Configuration

These invocation_tree configurations are available for an Invocation_Tree objects:

tree = ivt.Invocation_Tree()

• tree.filename : str
  • filename to save the tree to, defaults to 'tree.pdf'
• tree.show : bool
  • if True the default application is open to view 'tree.filename'
• tree.block : bool
  • if True program execution is blocked after the tree is saved
• tree.src_loc : bool
  • if True the source location is printed when blocking
• tree.each_line : bool
  • if True each line of the program is stepped through
• tree.max_string_len : int
  • the maximum string length, only the end is shown of longer strings
• tree.gifcount : int
  • if >=0 the out filename is numbered for animated gif making
• tree.indent : string
  • the string used for identing the local variables
• tree.color_active : string
  • HTML color for active function
• tree.color_paused* : string
  • HTML color for paused functions
• tree.color_returned*: string
  • HTML color for returned functions
• tree.hide : set()
  • set of all variables names that are not shown in the tree
• tree.to_string : dict[str, fun]
  • mapping from type/name to a to_string() function for custom printing of values

For convenience we provide these functions to set common configurations:

• ivt.blocking(filename), blocks on function call and return
• ivt.blocking_each_change(filename), blocks on each change of value
• ivt.debugger(filename), non-blocking for use in debugger tool (open <filename> manually)
• ivt.gif(filename), generates many output files on function call and return for gif creation
• ivt.gif_each_change(filename), generates many output files on each change of value for gif creation
• ivt.non_blocking(filename), non-blocking on each function call and return

Troubleshooting

• Adobe Acrobat Reader doesn't refresh a PDF file when it changes on disk and blocks updates which results in an Could not open 'somefile.pdf' for writing : Permission denied error. One solution is to install a PDF reader that does refresh (Evince, Okular, SumatraPDF, ...) and set it as the default PDF reader. Another solution is to save the tree to a different Graphviz Output Format.

Memory_Graph Package

The invocation_tree package visualizes function calls at different moments in time. If instead you want a detailed visualization of your data at the current time, check out the memory_graph package.