stack_data

This is a library that extracts data from stack frames and tracebacks, particularly to display more useful tracebacks than the default.

You can install it from PyPI:

pip install stack_data

Basic usage

Here's some code we'd like to inspect:

def foo():
    result = []
    for i in range(5):
        row = []
        result.append(row)
        print_stack()
        for j in range(5):
            row.append(i * j)
    return result

Note that foo calls a function print_stack(). In reality we can imagine that an exception was raised at this line, or a debugger stopped there, but this is easy to play with directly. Here's a basic implementation:

import inspect
import stack_data


def print_stack():
    frame = inspect.currentframe().f_back
    frame_info = stack_data.FrameInfo(frame)
    print(f"{frame_info.code.co_name} at line {frame_info.lineno}")
    print("-----------")
    for line in frame_info.lines:
        print(f"{'-->' if line.is_current else '   '} {line.lineno:4} | {line.render()}")

(Beware that this has a major bug - it doesn't account for line gaps, which we'll learn about later)

The output of one call to print_stack() looks like:

foo at line 9
-----------
       6 | for i in range(5):
       7 |     row = []
       8 |     result.append(row)
-->    9 |     print_stack()
      10 |     for j in range(5):

The code for print_stack() is fairly self-explanatory. If you want to learn more details about a particular class or method I suggest looking through some docstrings. FrameInfo is a class that accepts either a frame or a traceback object and provides a bunch of nice attributes and properties (which are cached so you don't need to worry about performance). In particular frame_info.lines is a list of Line objects. line.render() returns the source code of that line suitable for display. Without any arguments it simply strips any common leading indentation. Later on we'll see a more powerful use for it.

You can see that frame_info.lines includes some lines of surrounding context. By default it includes 3 pieces of context before the main line and 1 piece after. We can configure the amount of context by passing options:

options = stack_data.Options(before=1, after=0)
frame_info = stack_data.FrameInfo(frame, options)

Then the output looks like:

foo at line 9
-----------
       8 | result.append(row)
-->    9 | print_stack()

Note that these parameters are not the number of lines before and after to include, but the number of pieces. A piece is a range of one or more lines in a file that should logically be grouped together. A piece contains either a single simple statement or a part of a compound statement (loops, if, try/except, etc) that doesn't contain any other statements. Most pieces are a single line, but a multi-line statement or if condition is a single piece. In the example above, all pieces are one line, because nothing is spread across multiple lines. If we change our code to include some multiline bits:

def foo():
    result = []
    for i in range(5):
        row = []
        result.append(
            row
        )
        print_stack()
        for j in range(
                5
        ):
            row.append(i * j)
    return result

and then run the original code with the default options, then the output is:

foo at line 11
-----------
       6 | for i in range(5):
       7 |     row = []
       8 |     result.append(
       9 |         row
      10 |     )
-->   11 |     print_stack()
      12 |     for j in range(
      13 |             5
      14 |     ):

Now lines 8-10 and lines 12-14 are each a single piece. Note that the output is essentially the same as the original in terms of the amount of code. The division of files into pieces means that the edge of the context is intuitive and doesn't crop out parts of statements or expressions. For example, if context was measured in lines instead of pieces, the last line of the above would be for j in range( which is much less useful.

However, if a piece is very long, including all of it could be cumbersome. For this, Options has a parameter max_lines_per_piece, which is 6 by default. Suppose we have a piece in our code that's longer than that:

        row = [
            1,
            2,
            3,
            4,
            5,
        ]

frame_info.lines will truncate this piece so that instead of 7 Line objects it will produce 5 Line objects and one LINE_GAP in the middle, making 6 objects in total for the piece. Our code doesn't currently handle gaps, so it will raise an exception. We can modify it like so:

    for line in frame_info.lines:
        if line is stack_data.LINE_GAP:
            print("       (...)")
        else:
            print(f"{'-->' if line.is_current else '   '} {line.lineno:4} | {line.render()}")

Now the output looks like:

foo at line 15
-----------
       6 | for i in range(5):
       7 |     row = [
       8 |         1,
       9 |         2,
       (...)
      12 |         5,
      13 |     ]
      14 |     result.append(row)
-->   15 |     print_stack()
      16 |     for j in range(5):

Alternatively, you can flip the condition around and check if isinstance(line, stack_data.Line):. Either way, you should always check for line gaps, or your code may appear to work at first but fail when it encounters a long piece.

Note that the executing piece, i.e. the piece containing the current line being executed (line 15 in this case) is never truncated, no matter how long it is.

The lines of context never stray outside frame_info.scope, which is the innermost function or class definition containing the current line. For example, this is the output for a short function which has neither 3 lines before nor 1 line after the current line:

bar at line 6
-----------
       4 | def bar():
       5 |     foo()
-->    6 |     print_stack()

Sometimes it's nice to ensure that the function signature is always showing. This can be done with Options(include_signature=True). The result looks like this:

foo at line 14
-----------
       9 | def foo():
       (...)
      11 |     for i in range(5):
      12 |         row = []
      13 |         result.append(row)
-->   14 |         print_stack()
      15 |         for j in range(5):

To avoid wasting space, pieces never start or end with a blank line, and blank lines between pieces are excluded. So if our code looks like this:

    for i in range(5):
        row = []

        result.append(row)
        print_stack()

        for j in range(5):

The output doesn't change much, except you can see jumps in the line numbers:

      11 |     for i in range(5):
      12 |         row = []
      14 |         result.append(row)
-->   15 |         print_stack()
      17 |         for j in range(5):

Variables

You can also inspect variables and other expressions in a frame, e.g:

    for var in frame_info.variables:
        print(f"{var.name} = {repr(var.value)}")

which may output:

result = [[0, 0, 0, 0, 0], [0, 1, 2, 3, 4], [0, 2, 4, 6, 8], [0, 3, 6, 9, 12], []]
i = 4
row = []
j = 4

frame_info.variables returns a list of Variable objects, which have attributes name, value, and nodes, which is a list of all AST representing that expression.

A Variable may refer to an expression other than a simple variable name. It can be any expression evaluated by the library pure_eval which it deems 'interesting' (see those docs for more info). This includes expressions like foo.bar or foo[bar]. In these cases name is the source code of that expression. pure_eval ensures that it only evaluates expressions that won't have any side effects, e.g. where foo.bar is a normal attribute rather than a descriptor such as a property.

frame_info.variables is a list of all the interesting expressions found in frame_info.scope, e.g. the current function, which may include expressions not visible in frame_info.lines. You can restrict the list by using frame_info.variables_in_lines or even frame_info.variables_in_executing_piece. For more control you can use frame_info.variables_by_lineno. See the docstrings for more information.

Rendering lines with ranges and markers

Sometimes you may want to insert special characters into the text for display purposes, e.g. HTML or ANSI color codes. stack_data provides a few tools to make this easier.

Let's say we have a Line object where line.text (the original raw source code of that line) is "foo = bar", so line.text[6:9] is "bar", and we want to emphasise that part by inserting HTML at positions 6 and 9 in the text. Here's how we can do that directly:

markers = [
    stack_data.MarkerInLine(position=6, is_start=True, string="<b>"),
    stack_data.MarkerInLine(position=9, is_start=False, string="</b>"),
]
line.render(markers)  # returns "foo = <b>bar</b>"

Here is_start=True indicates that the marker is the first of a pair. This helps line.render() sort and insert the markers correctly so you don't end up with malformed HTML like foo<b>.<i></b>bar</i> where tags overlap.

Since we're inserting HTML, we should actually use line.render(markers, escape_html=True) which will escape special HTML characters in the Python source (but not the markers) so for example foo = bar < spam would be rendered as foo = <b>bar</b> &lt; spam.

Usually though you wouldn't create markers directly yourself. Instead you would start with one or more ranges and then convert them, like so:

ranges = [
    stack_data.RangeInLine(start=0, end=3, data="foo"),
    stack_data.RangeInLine(start=6, end=9, data="bar"),
]

def convert_ranges(r):
    if r.data == "bar":
        return "<b>", "</b>"        

# This results in `markers` being the same as in the above example.
markers = stack_data.markers_from_ranges(ranges, convert_ranges)

RangeInLine has a data attribute which can be any object. markers_from_ranges accepts a converter function to which it passes all the RangeInLine objects. If the converter function returns a pair of strings, it creates two markers from them. Otherwise it should return None to indicate that the range should be ignored, as with the first range containing "foo" in this example.

The reason this is useful is because there are built in tools to create these ranges for you. For example, if we change our print_stack() function to contain this:

def convert_variable_ranges(r):
    variable, _node = r.data
    return f'<span data-value="{repr(variable.value)}">', '</span>'

markers = stack_data.markers_from_ranges(line.variable_ranges, convert_variable_ranges)
print(f"{'-->' if line.is_current else '   '} {line.lineno:4} | {line.render(markers, escape_html=True)}")

Then the output becomes:

foo at line 15
-----------
       9 | def foo():
       (...)
      11 |     for <span data-value="4">i</span> in range(5):
      12 |         <span data-value="[]">row</span> = []
      14 |         <span data-value="[[0, 0, 0, 0, 0], [0, 1, 2, 3, 4], [0, 2, 4, 6, 8], [0, 3, 6, 9, 12], []]">result</span>.append(<span data-value="[]">row</span>)
-->   15 |         print_stack()
      17 |         for <span data-value="4">j</span> in range(5):

line.variable_ranges is a list of RangeInLines for each Variable that appears at least partially in this line. The data attribute of the range is a pair (variable, node) where node is the particular AST node from the list variable.nodes that corresponds to this range.

You can also use line.token_ranges (e.g. if you want to do your own syntax highlighting) or line.executing_node_ranges if you want to highlight the currently executing node identified by the executing library. Or if you want to make your own range from an AST node, use line.range_from_node(node, data). See the docstrings for more info.

Syntax highlighting with Pygments

If you'd like pretty colored text without the work, you can let Pygments do it for you. Just follow these steps:

1. pip install pygments separately as it's not a dependency of stack_data.
2. Create a pygments formatter object such as HtmlFormatter or Terminal256Formatter.
3. Pass the formatter to Options in the argument pygments_formatter.
4. Use line.render(pygmented=True) to get your formatted text. In this case you can't pass any markers to render.

If you want, you can also highlight the executing node in the frame in combination with the pygments syntax highlighting. For this you will need:

1. A pygments style - either a style class or a string that names it. See the documentation on styles and the styles gallery.
2. A modification to make to the style for the executing node, which is a string such as "bold" or "bg:#ffff00" (yellow background). See the documentation on style rules.
3. Pass these two things to stack_data.style_with_executing_node(style, modifier) to get a new style class.
4. Pass the new style to your formatter when you create it.

Note that this doesn't work with TerminalFormatter which just uses the basic ANSI colors and doesn't use the style passed to it in general.

Getting the full stack

Currently print_stack() doesn't actually print the stack, it just prints one frame. Instead of frame_info = FrameInfo(frame, options), let's do this:

for frame_info in FrameInfo.stack_data(frame, options):

Now the output looks something like this:

<module> at line 18
-----------
      14 |         for j in range(5):
      15 |             row.append(i * j)
      16 |     return result
-->   18 | bar()

bar at line 5
-----------
       4 | def bar():
-->    5 |     foo()

foo at line 13
-----------
      10 | for i in range(5):
      11 |     row = []
      12 |     result.append(row)
-->   13 |     print_stack()
      14 |     for j in range(5):

However, just as frame_info.lines doesn't always yield Line objects, FrameInfo.stack_data doesn't always yield FrameInfo objects, and we must modify our code to handle that. Let's look at some different sample code:

def factorial(x):
    return x * factorial(x - 1)


try:
    print(factorial(5))
except:
    print_stack()

In this code we've forgotten to include a base case in our factorial function so it will fail with a RecursionError and there'll be many frames with similar information. Similar to the built in Python traceback, stack_data avoids showing all of these frames. Instead you will get a RepeatedFrames object which summarises the information. See its docstring for more details.

Here is our updated implementation:

def print_stack():
    for frame_info in FrameInfo.stack_data(sys.exc_info()[2]):
        if isinstance(frame_info, FrameInfo):
            print(f"{frame_info.code.co_name} at line {frame_info.lineno}")
            print("-----------")
            for line in frame_info.lines:
                print(f"{'-->' if line.is_current else '   '} {line.lineno:4} | {line.render()}")

            for var in frame_info.variables:
                print(f"{var.name} = {repr(var.value)}")

            print()
        else:
            print(f"... {frame_info.description} ...\n")

And the output:

<module> at line 9
-----------
       4 | def factorial(x):
       5 |     return x * factorial(x - 1)
       8 | try:
-->    9 |     print(factorial(5))
      10 | except:

factorial at line 5
-----------
       4 | def factorial(x):
-->    5 |     return x * factorial(x - 1)
x = 5

factorial at line 5
-----------
       4 | def factorial(x):
-->    5 |     return x * factorial(x - 1)
x = 4

... factorial at line 5 (996 times) ...

factorial at line 5
-----------
       4 | def factorial(x):
-->    5 |     return x * factorial(x - 1)
x = -993

In addition to handling repeated frames, we've passed a traceback object to FrameInfo.stack_data instead of a frame.

If you want, you can pass collapse_repeated_frames=False to FrameInfo.stack_data (not to Options) and it will just yield FrameInfo objects for the full stack.