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Source code: Lib/dis.py
The dis module supports the analysis of CPython bytecode by disassembling it. The CPython bytecode which this module takes as an input is defined in the file Include/opcode.h and used by the compiler and the interpreter.
CPython implementation detail: Bytecode is an implementation detail of the CPython interpreter. No guarantees are made that bytecode will not be added, removed, or changed between versions of Python. Use of this module should not be considered to work across Python VMs or Python releases.
Example: Given the function myfunc():
def myfunc(alist):
return len(alist)
the following command can be used to display the disassembly of myfunc():
>>> dis.dis(myfunc)
2 0 LOAD_GLOBAL 0 (len)
3 LOAD_FAST 0 (alist)
6 CALL_FUNCTION 1
9 RETURN_VALUE
(The “2” is a line number).
The bytecode analysis API allows pieces of Python code to be wrapped in a Bytecode object that provides easy access to details of the compiled code.
The bytecode operations of a piece of code
This is a convenient wrapper around many of the functions listed below. Instantiate it with a function, method, string of code, or a code object (as returned by compile()).
Iterating over this yields the bytecode operations as Instruction instances.
The compiled code object.
Return a formatted multi-line string with detailed information about the code object, like code_info().
New in version 3.4.
Example:
>>> bytecode = dis.Bytecode(myfunc)
>>> for instr in bytecode:
... print(instr.opname)
...
LOAD_GLOBAL
LOAD_FAST
CALL_FUNCTION
RETURN_VALUE
The dis module also defines the following analysis functions that convert the input directly to the desired output. They can be useful if only a single operation is being performed, so the intermediate analysis object isn’t useful:
Return a formatted multi-line string with detailed code object information for the supplied function, method, source code string or code object.
Note that the exact contents of code info strings are highly implementation dependent and they may change arbitrarily across Python VMs or Python releases.
New in version 3.2.
Print detailed code object information for the supplied function, method, source code string or code object to file (or sys.stdout if file is not specified).
This is a convenient shorthand for print(code_info(x), file=file), intended for interactive exploration at the interpreter prompt.
New in version 3.2.
Changed in version 3.4: Added file parameter
Disassemble the x object. x can denote either a module, a class, a method, a function, a code object, a string of source code or a byte sequence of raw bytecode. For a module, it disassembles all functions. For a class, it disassembles all methods. For a code object or sequence of raw bytecode, it prints one line per bytecode instruction. Strings are first compiled to code objects with the compile() built-in function before being disassembled. If no object is provided, this function disassembles the last traceback.
The disassembly is written as text to the supplied file argument if provided and to sys.stdout otherwise.
Changed in version 3.4: Added file parameter
Disassemble the top-of-stack function of a traceback, using the last traceback if none was passed. The instruction causing the exception is indicated.
The disassembly is written as text to the supplied file argument if provided and to sys.stdout otherwise.
Changed in version 3.4: Added file parameter
Disassemble a code object, indicating the last instruction if lasti was provided. The output is divided in the following columns:
The parameter interpretation recognizes local and global variable names, constant values, branch targets, and compare operators.
The disassembly is written as text to the supplied file argument if provided and to sys.stdout otherwise.
Changed in version 3.4: Added file parameter
Return an iterator over the instructions in the supplied function, method, source code string or code object.
The iterator generates a series of Instruction named tuples giving the details of each operation in the supplied code.
The given line_offset is added to the starts_line attribute of any instructions that start a new line.
New in version 3.4.
This generator function uses the co_firstlineno and co_lnotab attributes of the code object code to find the offsets which are starts of lines in the source code. They are generated as (offset, lineno) pairs.
Detect all offsets in the code object code which are jump targets, and return a list of these offsets.
The get_instructions() function and Bytecode class provide details of bytecode instructions as Instruction instances:
Details for a bytecode operation
numeric code for operation, corresponding to the opcode values listed below and the bytecode values in the Opcode collections.
human readable name for operation
numeric argument to operation (if any), otherwise None
resolved arg value (if known), otherwise same as arg
human readable description of operation argument
start index of operation within bytecode sequence
line started by this opcode (if any), otherwise None
True if other code jumps to here, otherwise False
New in version 3.4.
The Python compiler currently generates the following bytecode instructions.
General instructions
Do nothing code. Used as a placeholder by the bytecode optimizer.
Removes the top-of-stack (TOS) item.
Swaps the two top-most stack items.
Lifts second and third stack item one position up, moves top down to position three.
Duplicates the reference on top of the stack.
Duplicates the two references on top of the stack, leaving them in the same order.
Unary operations
Unary operations take the top of the stack, apply the operation, and push the result back on the stack.
Implements TOS = +TOS.
Implements TOS = -TOS.
Implements TOS = not TOS.
Implements TOS = ~TOS.
Implements TOS = iter(TOS).
Binary operations
Binary operations remove the top of the stack (TOS) and the second top-most stack item (TOS1) from the stack. They perform the operation, and put the result back on the stack.
Implements TOS = TOS1 ** TOS.
Implements TOS = TOS1 * TOS.
Implements TOS = TOS1 // TOS.
Implements TOS = TOS1 / TOS.
Implements TOS = TOS1 % TOS.
Implements TOS = TOS1 + TOS.
Implements TOS = TOS1 - TOS.
Implements TOS = TOS1[TOS].
Implements TOS = TOS1 << TOS.
Implements TOS = TOS1 >> TOS.
Implements TOS = TOS1 & TOS.
Implements TOS = TOS1 ^ TOS.
Implements TOS = TOS1 | TOS.
In-place operations
In-place operations are like binary operations, in that they remove TOS and TOS1, and push the result back on the stack, but the operation is done in-place when TOS1 supports it, and the resulting TOS may be (but does not have to be) the original TOS1.
Implements in-place TOS = TOS1 ** TOS.
Implements in-place TOS = TOS1 * TOS.
Implements in-place TOS = TOS1 // TOS.
Implements in-place TOS = TOS1 / TOS.
Implements in-place TOS = TOS1 % TOS.
Implements in-place TOS = TOS1 + TOS.
Implements in-place TOS = TOS1 - TOS.
Implements in-place TOS = TOS1 << TOS.
Implements in-place TOS = TOS1 >> TOS.
Implements in-place TOS = TOS1 & TOS.
Implements in-place TOS = TOS1 ^ TOS.
Implements in-place TOS = TOS1 | TOS.
Implements TOS1[TOS] = TOS2.
Implements del TOS1[TOS].
Miscellaneous opcodes
Implements the expression statement for the interactive mode. TOS is removed from the stack and printed. In non-interactive mode, an expression statement is terminated with POP_STACK.
Continues a loop due to a continue statement. target is the address to jump to (which should be a FOR_ITER instruction).
Calls set.add(TOS1[-i], TOS). Used to implement set comprehensions.
Calls list.append(TOS[-i], TOS). Used to implement list comprehensions.
Calls dict.setitem(TOS1[-i], TOS, TOS1). Used to implement dict comprehensions.
For all of the SET_ADD, LIST_APPEND and MAP_ADD instructions, while the added value or key/value pair is popped off, the container object remains on the stack so that it is available for further iterations of the loop.
Returns with TOS to the caller of the function.
Loads all symbols not starting with '_' directly from the module TOS to the local namespace. The module is popped after loading all names. This opcode implements from module import *.
Removes one block from the block stack. Per frame, there is a stack of blocks, denoting nested loops, try statements, and such.
Removes one block from the block stack. The popped block must be an exception handler block, as implicitly created when entering an except handler. In addition to popping extraneous values from the frame stack, the last three popped values are used to restore the exception state.
Terminates a finally clause. The interpreter recalls whether the exception has to be re-raised, or whether the function returns, and continues with the outer-next block.
Pushes builtins.__build_class__() onto the stack. It is later called by CALL_FUNCTION to construct a class.
This opcode performs several operations before a with block starts. First, it loads __exit__() from the context manager and pushes it onto the stack for later use by WITH_CLEANUP. Then, __enter__() is called, and a finally block pointing to delta is pushed. Finally, the result of calling the enter method is pushed onto the stack. The next opcode will either ignore it (POP_TOP), or store it in (a) variable(s) (STORE_FAST, STORE_NAME, or UNPACK_SEQUENCE).
Cleans up the stack when a with statement block exits. TOS is the context manager’s __exit__() bound method. Below TOS are 1–3 values indicating how/why the finally clause was entered:
In the last case, TOS(SECOND, THIRD, FOURTH) is called, otherwise TOS(None, None, None). In addition, TOS is removed from the stack.
If the stack represents an exception, and the function call returns a ‘true’ value, this information is “zapped” and replaced with a single WHY_SILENCED to prevent END_FINALLY from re-raising the exception. (But non-local gotos will still be resumed.)
All of the following opcodes expect arguments. An argument is two bytes, with the more significant byte last.
Implements name = TOS. namei is the index of name in the attribute co_names of the code object. The compiler tries to use STORE_FAST or STORE_GLOBAL if possible.
Implements del name, where namei is the index into co_names attribute of the code object.
Unpacks TOS into count individual values, which are put onto the stack right-to-left.
Implements assignment with a starred target: Unpacks an iterable in TOS into individual values, where the total number of values can be smaller than the number of items in the iterable: one the new values will be a list of all leftover items.
The low byte of counts is the number of values before the list value, the high byte of counts the number of values after it. The resulting values are put onto the stack right-to-left.
Implements TOS.name = TOS1, where namei is the index of name in co_names.
Implements del TOS.name, using namei as index into co_names.
Works as STORE_NAME, but stores the name as a global.
Works as DELETE_NAME, but deletes a global name.
Pushes co_consts[consti] onto the stack.
Pushes the value associated with co_names[namei] onto the stack.
Creates a tuple consuming count items from the stack, and pushes the resulting tuple onto the stack.
Works as BUILD_TUPLE, but creates a list.
Works as BUILD_TUPLE, but creates a set.
Pushes a new dictionary object onto the stack. The dictionary is pre-sized to hold count entries.
Replaces TOS with getattr(TOS, co_names[namei]).
Performs a Boolean operation. The operation name can be found in cmp_op[opname].
Imports the module co_names[namei]. TOS and TOS1 are popped and provide the fromlist and level arguments of __import__(). The module object is pushed onto the stack. The current namespace is not affected: for a proper import statement, a subsequent STORE_FAST instruction modifies the namespace.
Loads the attribute co_names[namei] from the module found in TOS. The resulting object is pushed onto the stack, to be subsequently stored by a STORE_FAST instruction.
Increments bytecode counter by delta.
If TOS is true, sets the bytecode counter to target. TOS is popped.
If TOS is false, sets the bytecode counter to target. TOS is popped.
If TOS is true, sets the bytecode counter to target and leaves TOS on the stack. Otherwise (TOS is false), TOS is popped.
If TOS is false, sets the bytecode counter to target and leaves TOS on the stack. Otherwise (TOS is true), TOS is popped.
Set bytecode counter to target.
TOS is an iterator. Call its __next__() method. If this yields a new value, push it on the stack (leaving the iterator below it). If the iterator indicates it is exhausted TOS is popped, and the byte code counter is incremented by delta.
Loads the global named co_names[namei] onto the stack.
Pushes a block for a loop onto the block stack. The block spans from the current instruction with a size of delta bytes.
Pushes a try block from a try-except clause onto the block stack. delta points to the first except block.
Pushes a try block from a try-except clause onto the block stack. delta points to the finally block.
Store a key and value pair in a dictionary. Pops the key and value while leaving the dictionary on the stack.
Pushes a reference to the local co_varnames[var_num] onto the stack.
Stores TOS into the local co_varnames[var_num].
Deletes local co_varnames[var_num].
Pushes a reference to the cell contained in slot i of the cell and free variable storage. The name of the variable is co_cellvars[i] if i is less than the length of co_cellvars. Otherwise it is co_freevars[i - len(co_cellvars)].
Loads the cell contained in slot i of the cell and free variable storage. Pushes a reference to the object the cell contains on the stack.
Much like LOAD_DEREF but first checks the locals dictionary before consulting the cell. This is used for loading free variables in class bodies.
Stores TOS into the cell contained in slot i of the cell and free variable storage.
Empties the cell contained in slot i of the cell and free variable storage. Used by the del statement.
Raises an exception. argc indicates the number of parameters to the raise statement, ranging from 0 to 3. The handler will find the traceback as TOS2, the parameter as TOS1, and the exception as TOS.
Calls a function. The low byte of argc indicates the number of positional parameters, the high byte the number of keyword parameters. On the stack, the opcode finds the keyword parameters first. For each keyword argument, the value is on top of the key. Below the keyword parameters, the positional parameters are on the stack, with the right-most parameter on top. Below the parameters, the function object to call is on the stack. Pops all function arguments, and the function itself off the stack, and pushes the return value.
Pushes a new function object on the stack. TOS is the qualified name of the function; TOS1 is the code associated with the function. The function object is defined to have argc default parameters, which are found below TOS1.
Creates a new function object, sets its __closure__ slot, and pushes it on the stack. TOS is the qualified name of the function, TOS1 is the code associated with the function, and TOS2 is the tuple containing cells for the closure’s free variables. The function also has argc default parameters, which are found below the cells.
Pushes a slice object on the stack. argc must be 2 or 3. If it is 2, slice(TOS1, TOS) is pushed; if it is 3, slice(TOS2, TOS1, TOS) is pushed. See the slice() built-in function for more information.
Prefixes any opcode which has an argument too big to fit into the default two bytes. ext holds two additional bytes which, taken together with the subsequent opcode’s argument, comprise a four-byte argument, ext being the two most-significant bytes.
Calls a function. argc is interpreted as in CALL_FUNCTION. The top element on the stack contains the variable argument list, followed by keyword and positional arguments.
Calls a function. argc is interpreted as in CALL_FUNCTION. The top element on the stack contains the keyword arguments dictionary, followed by explicit keyword and positional arguments.
Calls a function. argc is interpreted as in CALL_FUNCTION. The top element on the stack contains the keyword arguments dictionary, followed by the variable-arguments tuple, followed by explicit keyword and positional arguments.
This is not really an opcode. It identifies the dividing line between opcodes which don’t take arguments < HAVE_ARGUMENT and those which do >= HAVE_ARGUMENT.
These collections are provided for automatic introspection of bytecode instructions:
Sequence of operation names, indexable using the bytecode.
Dictionary mapping operation names to bytecodes.
Sequence of all compare operation names.
Sequence of bytecodes that have a constant parameter.
Sequence of bytecodes that access a free variable (note that ‘free’ in this context refers to names in the current scope that are referenced by inner scopes or names in outer scopes that are referenced from this scope. It does not include references to global or builtin scopes).
Sequence of bytecodes that access an attribute by name.
Sequence of bytecodes that have a relative jump target.
Sequence of bytecodes that have an absolute jump target.
Sequence of bytecodes that access a local variable.
Sequence of bytecodes of Boolean operations.