Develop Reference

Python how to replace string through dictionary

EX)
import re
nodeDic = {"ABC-3K":"True","TTR":"True"}<br />
address = "ABC-3K and TTR"
result = eval(re.sub(r"\b(\w+)\b", lambda m: nodeDic.get(m.group(1), m.group(1)), address))<br />
print(result)
--->True and True
but I have errors:
Traceback (most recent call last): File
"C:/Users/IT1_807_6/PycharmProjects/choonlog/Test/eval.py", line 6, in
result = eval(re.sub(r"\b(\w+)\b", lambda m: nodeDic.get(m.group(1), m.group(1)), address)) File "", line
1
GSK-3b and True
^ SyntaxError: invalid syntax
When I remove hyphen(-) no matters. But when I use hyphen(-) symbol it matters. I have to use the hyphen(-)

Ok i found a regex that does what you want, but it is a complete mess:
(?:(?<=\A)|(?<=\s))(.+?)(?=\Z|\s)
If you use this regex:
nodeDic = {"ABC-3K":"True","TTR":"True"}
address = "ABC-3K and TTR"
result = eval(re.sub(r"(?:(?<=\A)|(?<=\s))(.+?)(?=\Z|\s)", lambda m: nodeDic.get(m.group(1),m.group(1)), address))
print(result) # prints True

Python 3: Check if a string is an import command

I want to check a string - is it an import command? I have tried
# Helper - analyses a string - is it an import string?
"""
fromlike - from foo import bar
classic - import foo
classic_as - import foo as baz
"""
def check_is_import(string):
importname = ''
fromlike = False
classic = False
classic_as = False
if string[0:4] is 'from':
fromlike = True
importname = ''
if not fromlike and (string[0:6] is 'import'):
classic = True
importname = string.split(' ')[1]
if classic:
commandlist = string.split(' ')
if commandlist[2] is 'as':
classic_as = True
importname = commandlist[3]
del commandlist
if fromlike:
return ('fromlike', importname)
elif classic and (not classic_as):
return ('classic', importname)
elif classic_as:
return ('classic_as', importname)
else:
return ('no_import', importname)
but it worked for "fromlike" imports. (Note: I'm not asking "why does this code don't work?", I'm just searching a solution) What code will sure detect all imports? Basically my code takes a slice of the string. If the [0:4] slice equals 'from', the string is a "fromlike import". Else: if the [0:6] slice equals 'import', the string is a "classic import". If it detects 'as', it will find the pseudo-name. This function must return a tuple which contains the import type under index 0 and imported module-name under index 1.

If you want to be sure to handle all Python import forms, have Python do the parsing. Use the ast.parse() function and use the resulting parse tree; you'll either get Import or ImportFrom objects:
| Import(alias* names)
| ImportFrom(identifier? module, alias* names, int? level)
Each alias consists of a name and optional identifier used to import the name as:
-- import name with optional 'as' alias.
alias = (identifier name, identifier? asname)
Note that there can be multiple imports! You either have classic or fromlike imports, and both can import multiple names. Your function needs to return a list of (type, name) tuples. For invalid inputs, raise an exception (ValueError is a good fit here):
import ast
def check_is_import(string):
try:
body = ast.parse(string).body
except SyntaxError:
# not valid Python
raise ValueError('No import found')
if len(body) > 1:
# not a single statement
raise ValueError('Multiple statements found')
if not isinstance(body[0], (ast.Import, ast.ImportFrom)):
raise ValueError('No import found')
type_ = 'classic' if isinstance(body[0], ast.Import) else 'fromlike'
results = []
for alias in body[0].names:
alias_type = type_
if alias.asname:
alias_type += '_as'
results.append((alias_type, alias.asname or alias.name))
return results
The method should probably be renamed to extract_import_names(), as that reflects what it does much better.
Demo:
>>> check_is_import('from foo import bar')
[('fromlike', 'bar')]
>>> check_is_import('import foo')
[('classic', 'foo')]
>>> check_is_import('import foo as baz')
[('classic_as', 'baz')]
>>> check_is_import('from foo import bar, baz as spam, monty as python')
[('fromlike', 'bar'), ('fromlike_as', 'spam'), ('fromlike_as', 'python')]
>>> check_is_import('import foo as baz, baz, spam as ham')
[('classic_as', 'baz'), ('classic', 'baz'), ('classic_as', 'ham')]
>>> check_is_import('invalid python')
Traceback (most recent call last):
File "<stdin>", line 3, in check_is_import
File "/Users/mjpieters/Development/Library/buildout.python/parts/opt/lib/python3.6/ast.py", line 35, in parse
return compile(source, filename, mode, PyCF_ONLY_AST)
File "<unknown>", line 1
invalid python
^
SyntaxError: invalid syntax
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in check_is_import
ValueError: No import found
>>> check_is_import('import foo; import bar')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 9, in check_is_import
ValueError: Multiple statements found
>>> check_is_import('1 + 1 == 2')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 11, in check_is_import
ValueError: No import found

Python Pyparsing Optional field

I am currently using pyparsing module to create my own interpreter. My current code is
import pyparsing as pp
# To parse a packet with srcip,dstip,srcmac,dstmac
identifier = pp.Word(pp.alphas,pp.alphanums+'_')
ipfield = pp.Word(pp.nums,max=3)
ipAddr = pp.Combine(ipfield+"."+ipfield+"."+ipfield+"."+ipfield)
hexint = pp.Word(pp.hexnums,exact=2)
macAddr = pp.Combine(hexint+(":"+hexint)*5)
ip = pp.Combine(identifier+"="+ipAddr)
mac = pp.Combine(identifier+"="+macAddr)
pkt = ip & ip & mac & mac
arg = "createpacket<" + pp.Optional(pkt) + ">"
arg.parseString("createpacket<srcip=192.168.1.3dstip=192.168.1.4srcmac=00:FF:FF:FF:FF:00>")
While I run the last line of the code to parse an example string I get an error as follows:
File "<stdin>", line 1, in <module>
File "/usr/lib/python2.7/dist-packages/pyparsing.py", line 1041, in parseString
raise exc
pyparsing.ParseException: Expected ">" (at char 13), (line:1, col:14)
Can anyone explain the reason for this error?

Python pyparsing issue

I am very new to python and using pyparsing but getting some exception with following code
while site_contents.find('---', line_end) != line_end + 2:
cut_start = site_contents.find(" ", site_contents.find("\r\n", start))
cut_end = site_contents.find(" ", cut_start+1)
line_end = site_contents.find("\r\n", cut_end)
name = site_contents[cut_start:cut_end].strip()
float_num = Word(nums + '.').setParseAction(lambda t:float(t[0]))
nonempty_line = Literal(name) + Word(nums+',') + float_num + Suppress(Literal('-')) + float_num * 2
empty_line = Literal(name) + Literal('-')
line = nonempty_line | empty_line
parsed = line.parseString(site_contents[cut_start:line_end])
start = line_end
Exception
Traceback (most recent call last):
File "D:\Ecllipse_Python\HellloWorld\src\HelloPython.py", line 108, in <module>
parsed = line.parseString(site_contents[cut_start:line_end]) # parse line of data following cut name
File "C:\Users\arbatra\AppData\Local\Continuum\Anaconda\lib\site-packages\pyparsing.py", line 1041, in parseString
raise exc
pyparsing.ParseException: Expected W:(0123...) (at char 38), (line:1, col:39)
how to resolve this issue?

You'll get a little better exception message if you give names to your expressions, using setName. From the "Expected W:(0123...)" part of the exception message, it looks like the parser is not finding a numeric value where it is expected. But the default name is not showing us enough to know which type of numeric field is expected. Modify your parser to add setName as shown below, and also change the defintion of nonempty_line:
float_num = Word(nums + '.').setParseAction(lambda t:float(t[0])).setName("float_num")
integer_with_commas = Word(nums + ',').setName("int_with_commas")
nonempty_line = Literal(name) + integer_with_commas + float_num + Suppress(Literal('-')) + float_num * 2
I would also preface the call to parseString with:
print site_contents[cut_start:line_end]
at least while you are debugging. Then you can compare the string being parsed with the error message, including the column number where the parse error is occurring, as given in your posted example as "(at char 38), (line:1, col:39)". "char xx" starts with the first character as "char 0"; "col:xx" starts with the first column as "col:1".
These code changes might help you pinpoint your problem:
print "12345678901234567890123456789012345678901234567890"
print site_contents[cut_start:line_end]
try:
parsed = line.parseString(site_contents[cut_start:line_end])
except ParseException as pe:
print pe.loc*' ' + '^'
print pe
Be sure to run this in a window that uses a monospaced font (so that all the character columns line up, and all characters are the same width as each other).
Once you've done this, you may have enough information to fix the problem yourself, or you'll have some better output to edit into your original question so we can help you better.

Implementing C-like assert

I'm trying to implement an assert function. How can I get the text of the failing condition into the error message? If I have to parse it from the backtrace, can I portably rely on anything about the format of frames?

AssertionError is just like any other exception in python, and assert is a simple statement that is equivalent to
if __debug__:
if not expression: raise AssertionError
or
if __debug__:
if not expression1: raise AssertionError(expression2)
so you can add a second parameter to your assertion to have additional output
from sys import exc_info
from traceback import print_exception
# assertions are simply exceptions in Python
try:
assert False, "assert was false"
except AssertionError:
print_exception(*exc_info())
outputs
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
AssertionError: assert was false

If you're sure the expression to test is secure you could do something like this:
File my_assert.py:
import sys
def my_assert(condition):
caller = sys._getframe(1)
if not eval(condition, caller.f_globals, caller.f_locals):
raise AssertionError(repr(condition) + " on line " +
str(caller.f_lineno) + ' in ' +
caller.f_code.co_name)
File test_my_assert.py:
from my_assert import my_assert
global_var = 42
def test():
local_var = 17
my_assert('local_var*2 < global_var') # OK
my_assert('local_var > global_var')
test()
Output:
Traceback (most recent call last):
File "test_my_assert.py", line 10, in <module>
test()
File "test_my_assert.py", line 8, in test
my_assert('local_var > global_var')
File "my_assert.py", line 8, in my_assert
caller.f_code.co_name)
AssertionError: 'local_var > global_var' on line 8 in test

My very hackish solution:
def my_assert(condition):
if not eval(condition):
# error stuff
Then use it by placing the condition in quotation marks. It is then a string that can be printed in the error message.
Or, if you want it to actually raise an AssertionError:
def my_assert(condition):
if not eval(condition):
raise AssertionError(condition)