lambda with condition x: 1/x if x!= 0 not work

Question

I want to draw an inverse proportional function with GraphScene, everything works, However when I set condition of x != 0, a SyntaxError pops:

f11 = self.get_graph(lambda x: 1/x if x!= 0)

SyntaxError: invalid syntax

the error indicates the last parenthesis

I searched a lot, the lambda x: 1/x if x!= 0 should be a correct python syntax, don't know why it not work! thanks for any help.

"should be a correct Python syntax" -- what specific thing that you read leads you to believe that? — Charles Duffy, Jul 24 '19 at 19:39

TheoremOfBeethoven · Answer 1 · 2019-07-24T20:10:39.477

Graphs are created using bezier curves, bezier curves cannot be discontinuous, therefore, you must create multiple graphs for each domain you want to use.

class AddingDomains(GraphScene):
    CONFIG = {
        "y_max" : 5,
        "y_min" : -5,
        "x_max" : 6,
        "x_min" : -6,
        "graph_origin": ORIGIN,
    }
    def construct(self):
        self.setup_axes()
        graph_left = self.get_graph(lambda x : 1/x,  
                                    color = GREEN,
                                    x_min = self.x_min, 
                                    x_max = 1/self.y_min
                                    )
        graph_right = self.get_graph(lambda x : 1/x,  
                                    color = GREEN,
                                    x_min = 1/self.y_max, 
                                    x_max = self.x_max
                                    )
        graph=VGroup(graph_left,graph_right)
        self.play(
            ShowCreation(graph),
            run_time = 2,
            rate_func= double_smooth
        )
        self.wait()

Or

class AddingDomains2(GraphScene):
    CONFIG = {
        "y_max" : 5,
        "y_min" : -5,
        "x_max" : 6,
        "x_min" : -6,
        "graph_origin": ORIGIN,
    }
    def construct(self):
        self.setup_axes()
        graph_left = self.get_graph(lambda x : 1/x,  
                                    color = GREEN,
                                    x_min = self.x_min, 
                                    x_max = 1/self.y_min
                                    )
        graph_right = self.get_graph(lambda x : 1/x,  
                                    color = GREEN,
                                    x_min = 1/self.y_max, 
                                    x_max = self.x_max
                                    )
        graph=VMobject(color=RED)
        graph.append_points(graph_left.points)
        graph.append_points(graph_right.points)
        self.play(
            ShowCreation(graph),
            run_time = 2,
            rate_func= double_smooth
        )
        self.wait()

Returns: More information in manimlib/mobject/types/vectorized_mobject.py and manimlib/mobject/functions.py.

Yes, because even if we use Charles Duffy's answer, the graph will not be displayed correctly. — TheoremOfBeethoven, Jul 24 '19 at 20:14
To be sure; I treated this as a Python question, not a `manim` question. I think there's value to having a answers addressing it both ways; between the two, we cover both the OP's misunderstandings about Python syntax, and their misunderstandings about `manim`. — Charles Duffy, Jul 24 '19 at 20:25
@TheoremOfBeethoven, this answer helps, but I have to choose Charles's as answer because the title, thank you! — James Hao, Jul 25 '19 at 02:58
I think it's the right answer, and as Charles said before, my answer is relative only to manim — TheoremOfBeethoven, Jul 25 '19 at 19:35

score 2 · Accepted Answer · answered Jul 24 '19 at 19:36

Add an else telling what the lambda should evaluate to when x==0, and suddenly you have valid syntax:

lambda x: 1/x if x != 0 else 0

This syntax construct was added in PEP-308, which was adopted in Python 2.5. From the PEP, the grammar changes are described as follows:

test: or_test ['if' or_test 'else' test] | lambdef
or_test: and_test ('or' and_test)*
...
testlist_safe: or_test [(',' or_test)+ [',']]
...
gen_for: 'for' exprlist 'in' or_test [gen_iter]

As you can see, else is mandatory; there's no way to have a test without both an if and an else.

lambda with condition x: 1/x if x!= 0 not work

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