{"id":4978,"date":"2024-07-12T18:33:03","date_gmt":"2024-07-12T22:33:03","guid":{"rendered":"https:\/\/ikriv.com\/blog\/?p=4978"},"modified":"2024-07-12T18:35:50","modified_gmt":"2024-07-12T22:35:50","slug":"python-eval-difference-between-globals-and-locals","status":"publish","type":"post","link":"https:\/\/ikriv.com\/blog\/?p=4978","title":{"rendered":"Python eval: difference between globals and locals"},"content":{"rendered":"

Python eval()<\/a><\/code> receives two optional dictionaries: globals<\/code> and locals<\/code>. I was finally able to find at least two differences between them.<\/p>\n

First difference: global variables survive until after the eval()<\/code> call, local variables do not. This becomes important, if eval()<\/code> returns a lambda.<\/p>\n

\r\nglobs = {"a": 42 }\r\nlocs = {"b": 10 }\r\nprint(eval("a+b", globs, locs))       # prints 52; no visible difference between usage of a and b\r\n\r\nf = eval("lambda: a", globs, locs)    # global variable 'a' survives the eval call and can be used in a lambda later\r\nprint(f())                            # prints 42\r\n\r\nglobs["a"] = 100                      # we can even change the value of a, and the lambda will pick it up\r\nprint(f())                            # prints 100\r\n\r\ndel globs["a"]                        # if we delete globs["a"] the lambda will stop working\r\nprint(f())                            # raises NameError: name 'a' is not defined\r\n\r\ng = eval("lambda: b", globs, locs)    # this succeeds, but local variable 'b' will not survive beyond the eval call\r\nprint(g())                            # raises NameError: name 'b' is not defined \r\n<\/pre>\n
Second difference: local variables override global variables with the same name. This, however, is applicable only during the eval()<\/code> call. If eval()<\/code>\u00a0returns a lambda, the global variable will be used when the lambda is invoked.<\/p>\n
\r\nglobs = {"a": 42 }\r\nlocs = {"a": 10 }\r\ndata = eval("(a, lambda: a)", globs, locs) # returns a tuple of a value and a lambda\r\nprint(data[0], data[1]())                  # prints "10 42"; lambda returns the value of the global variable\r\n<\/pre>\n
I could not find information about any other differences in the documentation or elsewhere on the Internet, and even those above I found through experiment. So, I am documenting them here for posterity.<\/p>\n","protected":false},"excerpt":{"rendered":"
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