## 3.12.1 Creating AST Objects

AST objects may be created from source code or from a parse tree. When creating an AST object from source, different functions are used to create the `'eval'` and `'exec'` forms.

`expr` (string)
  The `expr()` function parses the parameter `string` as if it were an input to `compile(string, 'eval')`. If the parse succeeds, an AST object is created to hold the internal parse tree representation, otherwise an appropriate exception is thrown.

`suite` (string)
  The `suite()` function parses the parameter `string` as if it were an input to `compile(string, 'exec')`. If the parse succeeds, an AST object is created to hold the internal parse tree representation, otherwise an appropriate exception is thrown.

`sequence2ast` (sequence)
  This function accepts a parse tree represented as a sequence and builds an internal representation if possible. If it can validate that the tree conforms to the Python grammar and all nodes are valid node types in the host version of Python, an AST object is created from the internal representation and returned to the called. If there is a problem creating the internal representation, or if the tree cannot be validated, a `ParserError` exception is thrown. An AST object created this way should not be assumed to compile correctly; normal exceptions thrown by compilation may still be initiated when the AST object is passed to `compileast()`. This may indicate problems not related to syntax (such as a `MemoryError` exception), but may also be due to constructs such as the result of parsing `del f(0)`, which escapes the Python parser but is checked by the bytecode compiler.

  Sequences representing terminal tokens may be represented as either two-element lists of the form `(1, 'name')` or as three-element lists of the form `(1, 'name', 56)`. If the third element is present, it is assumed to be a valid line number. The line number may be specified for any subset of the terminal symbols in the input tree.

`tuple2ast` (sequence)
  This is the same function as `sequence2ast()`. This entry point is maintained for backward compatibility.