showemptyattrs option on Node.show() #570

cellularmitosis · 2025-04-04T10:51:27Z

This PR adds a hideemptyattrs option to Node.show(). Let me know what you think!

Given hello.c:

int main(int argc, char** argv) {
    printf("Hello, world!\n");
    return 0;
}

Here's a comparison of all of the options.

default show():

ast.show(attrnames=False, hideemptyattrs=False, nodenames=False, showcoord=False):
FileAST: 
  FuncDef: 
    Decl: main, [], [], [], []
      FuncDecl: 
        ParamList: 
          Decl: argc, [], [], [], []
            TypeDecl: argc, [], None
              IdentifierType: ['int']
          Decl: argv, [], [], [], []
            PtrDecl: []
              PtrDecl: []
                TypeDecl: argv, [], None
                  IdentifierType: ['char']
        TypeDecl: main, [], None
          IdentifierType: ['int']
    Compound: 
      FuncCall: 
        ID: printf
        ExprList: 
          Constant: string, "Hello, world!\n"
      Return: 
        Constant: int, 0

with hideemptyattrs:

ast.show(attrnames=False, hideemptyattrs=True, nodenames=False, showcoord=False):
FileAST: 
  FuncDef: 
    Decl: main
      FuncDecl: 
        ParamList: 
          Decl: argc
            TypeDecl: argc
              IdentifierType: ['int']
          Decl: argv
            PtrDecl: 
              PtrDecl: 
                TypeDecl: argv
                  IdentifierType: ['char']
        TypeDecl: main
          IdentifierType: ['int']
    Compound: 
      FuncCall: 
        ID: printf
        ExprList: 
          Constant: string, "Hello, world!\n"
      Return: 
        Constant: int, 0

attrnames=True:

ast.show(attrnames=True, hideemptyattrs=False, nodenames=False, showcoord=False):
FileAST: 
  FuncDef: 
    Decl: name=main, quals=[], align=[], storage=[], funcspec=[]
      FuncDecl: 
        ParamList: 
          Decl: name=argc, quals=[], align=[], storage=[], funcspec=[]
            TypeDecl: declname=argc, quals=[], align=None
              IdentifierType: names=['int']
          Decl: name=argv, quals=[], align=[], storage=[], funcspec=[]
            PtrDecl: quals=[]
              PtrDecl: quals=[]
                TypeDecl: declname=argv, quals=[], align=None
                  IdentifierType: names=['char']
        TypeDecl: declname=main, quals=[], align=None
          IdentifierType: names=['int']
    Compound: 
      FuncCall: 
        ID: name=printf
        ExprList: 
          Constant: type=string, value="Hello, world!\n"
      Return: 
        Constant: type=int, value=0

with hideemptyattrs:

ast.show(attrnames=True, hideemptyattrs=True, nodenames=False, showcoord=False):
FileAST: 
  FuncDef: 
    Decl: name=main
      FuncDecl: 
        ParamList: 
          Decl: name=argc
            TypeDecl: declname=argc
              IdentifierType: names=['int']
          Decl: name=argv
            PtrDecl: 
              PtrDecl: 
                TypeDecl: declname=argv
                  IdentifierType: names=['char']
        TypeDecl: declname=main
          IdentifierType: names=['int']
    Compound: 
      FuncCall: 
        ID: name=printf
        ExprList: 
          Constant: type=string, value="Hello, world!\n"
      Return: 
        Constant: type=int, value=0

nodenames=True:

ast.show(attrnames=True, hideemptyattrs=False, nodenames=True, showcoord=False):
FileAST: 
  FuncDef <ext[0]>: 
    Decl <decl>: name=main, quals=[], align=[], storage=[], funcspec=[]
      FuncDecl <type>: 
        ParamList <args>: 
          Decl <params[0]>: name=argc, quals=[], align=[], storage=[], funcspec=[]
            TypeDecl <type>: declname=argc, quals=[], align=None
              IdentifierType <type>: names=['int']
          Decl <params[1]>: name=argv, quals=[], align=[], storage=[], funcspec=[]
            PtrDecl <type>: quals=[]
              PtrDecl <type>: quals=[]
                TypeDecl <type>: declname=argv, quals=[], align=None
                  IdentifierType <type>: names=['char']
        TypeDecl <type>: declname=main, quals=[], align=None
          IdentifierType <type>: names=['int']
    Compound <body>: 
      FuncCall <block_items[0]>: 
        ID <name>: name=printf
        ExprList <args>: 
          Constant <exprs[0]>: type=string, value="Hello, world!\n"
      Return <block_items[1]>: 
        Constant <expr>: type=int, value=0

with hideemptyattrs:

ast.show(attrnames=True, hideemptyattrs=True, nodenames=True, showcoord=False):
FileAST: 
  FuncDef <ext[0]>: 
    Decl <decl>: name=main
      FuncDecl <type>: 
        ParamList <args>: 
          Decl <params[0]>: name=argc
            TypeDecl <type>: declname=argc
              IdentifierType <type>: names=['int']
          Decl <params[1]>: name=argv
            PtrDecl <type>: 
              PtrDecl <type>: 
                TypeDecl <type>: declname=argv
                  IdentifierType <type>: names=['char']
        TypeDecl <type>: declname=main
          IdentifierType <type>: names=['int']
    Compound <body>: 
      FuncCall <block_items[0]>: 
        ID <name>: name=printf
        ExprList <args>: 
          Constant <exprs[0]>: type=string, value="Hello, world!\n"
      Return <block_items[1]>: 
        Constant <expr>: type=int, value=0

showcoord=True:

ast.show(attrnames=True, hideemptyattrs=False, nodenames=True, showcoord=True):
FileAST:  (at None)
  FuncDef <ext[0]>:  (at :1:5)
    Decl <decl>: name=main, quals=[], align=[], storage=[], funcspec=[] (at :1:5)
      FuncDecl <type>:  (at :1:5)
        ParamList <args>:  (at :1:14)
          Decl <params[0]>: name=argc, quals=[], align=[], storage=[], funcspec=[] (at :1:14)
            TypeDecl <type>: declname=argc, quals=[], align=None (at :1:14)
              IdentifierType <type>: names=['int'] (at :1:10)
          Decl <params[1]>: name=argv, quals=[], align=[], storage=[], funcspec=[] (at :1:25)
            PtrDecl <type>: quals=[] (at :1:25)
              PtrDecl <type>: quals=[] (at :1:24)
                TypeDecl <type>: declname=argv, quals=[], align=None (at :1:27)
                  IdentifierType <type>: names=['char'] (at :1:20)
        TypeDecl <type>: declname=main, quals=[], align=None (at :1:5)
          IdentifierType <type>: names=['int'] (at :1:1)
    Compound <body>:  (at :1:1)
      FuncCall <block_items[0]>:  (at :2:5)
        ID <name>: name=printf (at :2:5)
        ExprList <args>:  (at :2:12)
          Constant <exprs[0]>: type=string, value="Hello, world!\n" (at :2:12)
      Return <block_items[1]>:  (at :3:5)
        Constant <expr>: type=int, value=0 (at :3:12)

with hideemptyattrs:

ast.show(attrnames=True, hideemptyattrs=True, nodenames=True, showcoord=True):
FileAST:  (at None)
  FuncDef <ext[0]>:  (at :1:5)
    Decl <decl>: name=main (at :1:5)
      FuncDecl <type>:  (at :1:5)
        ParamList <args>:  (at :1:14)
          Decl <params[0]>: name=argc (at :1:14)
            TypeDecl <type>: declname=argc (at :1:14)
              IdentifierType <type>: names=['int'] (at :1:10)
          Decl <params[1]>: name=argv (at :1:25)
            PtrDecl <type>:  (at :1:25)
              PtrDecl <type>:  (at :1:24)
                TypeDecl <type>: declname=argv (at :1:27)
                  IdentifierType <type>: names=['char'] (at :1:20)
        TypeDecl <type>: declname=main (at :1:5)
          IdentifierType <type>: names=['int'] (at :1:1)
    Compound <body>:  (at :1:1)
      FuncCall <block_items[0]>:  (at :2:5)
        ID <name>: name=printf (at :2:5)
        ExprList <args>:  (at :2:12)
          Constant <exprs[0]>: type=string, value="Hello, world!\n" (at :2:12)
      Return <block_items[1]>:  (at :3:5)
        Constant <expr>: type=int, value=0 (at :3:12)

eliben · 2025-04-04T13:17:57Z

pycparser/c_ast.py

        pass

-    def show(self, buf=sys.stdout, offset=0, attrnames=False, nodenames=False, showcoord=False, _my_node_name=None):
+    def show(self, buf=sys.stdout, offset=0, attrnames=False, hideemptyattrs=False, nodenames=False, showcoord=False, _my_node_name=None):


In general, I think it's preferable to have positive logic as much as possible, to avoid double-negation in code and reasoning about code.

Can you make it showemptyattrs=True?

Hmm, yeah showemptyattrs does seem easier to reason about after extracting a helper.

if self.attr_names: is_empty = lambda v: v is None or (hasattr(v, '__len__') and len(v) == 0) nvlist = [(n, getattr(self,n)) for n in self.attr_names \ if showemptyattrs or not is_empty(getattr(self,n))]

Do you think it is worth also extracting a filter, or just leave it inline?

if self.attr_names: is_empty = lambda v: v is None or (hasattr(v, '__len__') and len(v) == 0) attr_filter = lambda v: showemptyattrs or not is_empty(v) nvlist = [(n, getattr(self,n)) for n in self.attr_names if attr_filter(getattr(self,n))]

eliben · 2025-04-04T13:19:37Z

pycparser/c_ast.py

            buf.write(lead + self.__class__.__name__+ ': ')

        if self.attr_names:
+            nvlist = []


If you factor out the emptiness check into a helper function, this loop can become a simple loop comprehension again:

nvlist = [(n, getattr(self, n)) for n in self.attr_names if ...]

Or something similar

cellularmitosis · 2025-04-04T15:41:25Z

Ok, new commit pushed.

cellularmitosis · 2025-04-05T21:26:05Z

Thanks for merging!

Just to have it indexed by google, I'll drop an example here about how users can patch in their own custom Node.show() implementation without forking pycparser. Here's a different show() style I came up with:

#!/usr/bin/env python3

# Demonstration of patching in an alternative Node.show() method.

import sys
from pycparser import c_parser
from pycparser.c_ast import *

def show(self, buf=sys.stdout, indent=4, showcoord=True, _my_node_name=None, _lead='', _lastcoord=None, _depth=1):
    """ Pretty print the Node and all its attributes and
        children (recursively) to a buffer.

        buf:
            Open IO buffer into which the Node is printed.

        indent:
            The number of spaces to indent at each level.
        
        showcoord:
            Do you want the coordinates of each Node to be
            displayed.

        returns the number of parens which need to be closed by the parent.
    """
    # print the node type
    s = _lead
    if _my_node_name:
        s += "%s = " % _my_node_name
    s += "%s(" % (self.__class__.__name__)
    if showcoord and self.coord:
        # only print the line number if it has changed
        coord_did_change = _lastcoord is None \
            or self.coord.file != _lastcoord.file \
            or self.coord.line != _lastcoord.line
        if coord_did_change:
            s += '  // '
            if self.coord.file:
                s += '%s ' % self.coord.file
            s += 'line %s' % self.coord.line
            _lastcoord = self.coord
    buf.write(s + '\n')

    # use dots to give visual column cues
    lead2 = _lead + '.' + (' ' * (indent-1))

    # print the attributes
    if self.attr_names:
        for name, value in [(n, getattr(self,n)) for n in self.attr_names]:
            # suppress empty fields
            if value is None:
                continue
            if hasattr(value, '__len__') and len(value) == 0:
                continue
            buf.write(lead2 + "%s = %s\n" % (name, value))

    # print the children.
    unclosed_depth = 0
    for i, (child_name, child) in enumerate(self.children()):
        is_last = i+1 == len(self.children())
        unclosed_depth = child.show(
            buf,
            indent=indent,
            _my_node_name=child_name,
            _lead=lead2,
            _lastcoord=_lastcoord,
            _depth=_depth+1
        )
        if not is_last:
            dot_spaces = ('.' + ' '*(indent-1))
            spaces_cparen = ')' + ' ' * (indent-1)
            buf.write((dot_spaces * _depth) + (spaces_cparen * unclosed_depth) + '\n')
        else:
            # let the caller close out parens after the last child
            pass

    if _depth == 1:
        # this is the final paren closing
        unclosed_depth += 1
        spaces_cparen = ')' + ' ' * (indent-1)
        buf.write((spaces_cparen * unclosed_depth) + '\n')
    else:
        # let the caller close out parens after the last child
        return unclosed_depth + 1


# patch in our custom show
Node.show = show


if __name__ == "__main__":
    if len(sys.argv) < 2:
        sys.stderr.write("Error: no filename given.\n")
        sys.exit(1)

    fname = sys.argv[1]
    with open(fname, 'r') as fd:
        text = fd.read()
    parser = c_parser.CParser()
    ast = parser.parse(text)
    ast.show()

given hello.c:

int main(int argc, char** argv) {
    printf("Hello, world!\n");
    return 0;
}

the above prints the following:

FileAST(
.   ext[0] = FuncDef(  // line 1
.   .   decl = Decl(
.   .   .   name = main
.   .   .   type = FuncDecl(
.   .   .   .   args = ParamList(
.   .   .   .   .   params[0] = Decl(
.   .   .   .   .   .   name = argc
.   .   .   .   .   .   type = TypeDecl(
.   .   .   .   .   .   .   declname = argc
.   .   .   .   .   .   .   type = IdentifierType(
.   .   .   .   .   .   .   .   names = ['int']
.   .   .   .   .   )   )   )   
.   .   .   .   .   params[1] = Decl(
.   .   .   .   .   .   name = argv
.   .   .   .   .   .   type = PtrDecl(
.   .   .   .   .   .   .   type = PtrDecl(
.   .   .   .   .   .   .   .   type = TypeDecl(
.   .   .   .   .   .   .   .   .   declname = argv
.   .   .   .   .   .   .   .   .   type = IdentifierType(
.   .   .   .   .   .   .   .   .   .   names = ['char']
.   .   .   .   )   )   )   )   )   )   
.   .   .   .   type = TypeDecl(
.   .   .   .   .   declname = main
.   .   .   .   .   type = IdentifierType(
.   .   .   .   .   .   names = ['int']
.   .   )   )   )   )   
.   .   body = Compound(
.   .   .   block_items[0] = FuncCall(  // line 2
.   .   .   .   name = ID(
.   .   .   .   .   name = printf
.   .   .   .   )   
.   .   .   .   args = ExprList(
.   .   .   .   .   exprs[0] = Constant(
.   .   .   .   .   .   type = string
.   .   .   .   .   .   value = "Hello, world!\n"
.   .   .   )   )   )   
.   .   .   block_items[1] = Return(  // line 3
.   .   .   .   expr = Constant(
.   .   .   .   .   type = int
.   .   .   .   .   value = 0
)   )   )   )   )

cellularmitosis added 2 commits April 4, 2025 05:42

hideemptyattrs option on Node.show()

53b501a

slight simplification

4bd5ae9

eliben reviewed Apr 4, 2025

View reviewed changes

prefer positive logic

d3e8424

cellularmitosis changed the title ~~hideemptyattrs option on Node.show()~~ showemptyattrs option on Node.show() Apr 4, 2025

eliben approved these changes Apr 5, 2025

View reviewed changes

eliben merged commit f04fdcd into eliben:main Apr 5, 2025
15 checks passed

simonlindholm mentioned this pull request Sep 15, 2025

Update _ast_gen.py to be in sync with c_ast.py #582

Merged

never147 mentioned this pull request Oct 28, 2025

Bump pycparser from 2.22 to 2.23 never147/ansible-desktop#62

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

showemptyattrs option on Node.show() #570

showemptyattrs option on Node.show() #570

Uh oh!

cellularmitosis commented Apr 4, 2025 •

edited

Loading

Uh oh!

eliben Apr 4, 2025

Uh oh!

cellularmitosis Apr 4, 2025 •

edited

Loading

Uh oh!

eliben Apr 4, 2025

Uh oh!

cellularmitosis commented Apr 4, 2025

Uh oh!

Uh oh!

cellularmitosis commented Apr 5, 2025 •

edited

Loading

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

showemptyattrs option on Node.show() #570

showemptyattrs option on Node.show() #570

Uh oh!

Conversation

cellularmitosis commented Apr 4, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

eliben Apr 4, 2025

Choose a reason for hiding this comment

Uh oh!

cellularmitosis Apr 4, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

eliben Apr 4, 2025

Choose a reason for hiding this comment

Uh oh!

cellularmitosis commented Apr 4, 2025

Uh oh!

Uh oh!

cellularmitosis commented Apr 5, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

cellularmitosis commented Apr 4, 2025 •

edited

Loading

cellularmitosis Apr 4, 2025 •

edited

Loading

cellularmitosis commented Apr 5, 2025 •

edited

Loading