Negative float Problem - Python (Tkinter)

Question

Ich habe Probleme mit einer Zeile meines Codes, wo ich positive Zahlen tun kann, aber ich kann kein - Zeichen vor den Zahlen hinzufügen, um es zu einer negativen Zahl zu machen. Hier

ist der Code:

as_meter = dict(mm=0.001, cm=0.01, inch=0.0254,ft=0.3048, yd=0.9144, m=1.0,km=1000.0, mi=1609.344,) 
def update_length(self, *args): 
    try: 
     v = float(self.Text_Length_Left.get()) 
    except ValueError: 
     self.Text_Length_Left.set('') 
     self.Text_Length_Right.set('') 
     return 

Hier ist die linke Eingabefeld Code, in dem Sie den Schwimmer ein:

self.Text_Length_Left = StringVar() 
self.Entry_Length_Left = ttk.Entry(self.length_frame, textvariable = self.Text_Length_Left,width = 13) 
self.Text_Length_Left.trace('w', self.update_length) 
self.Entry_Length_Left.grid(row = 3, column = 0, padx = 5, pady = 5) 

Vielen Dank für jede Hilfe!

bearbeiten

-Code, um sie auszuführen:

class Conversion: 

as_meter = dict(mm=0.001, cm=0.01, inch=0.0254,ft=0.3048, yd=0.9144, m=1.0,km=1000.0, mi=1609.344,) 
def update_length(self, *args): 
    try: 
     v = float(self.Text_Length_Left.get()) 
    except ValueError: 
     self.Text_Length_Left.set('') 
     self.Text_Length_Right.set('') 
     return 
    m = v * self.as_meter [self.measurements.get()] 
    r = m/self.as_meter[self.measurements1.get()] 
    self.Text_Length_Right.set("{:.5g}".format(r)) 

class GUI(Conversion): 
def __init__(self,root): 
    self.notebook = ttk.Notebook(root) 
    self.notebook.pack() 

    self.length_frame = ttk.Frame(self.notebook) 
    self.weight_frame = ttk.Frame(self.notebook) 
    self.temperature_frame = ttk.Frame(self.notebook) 
    self.help_frame = ttk.Frame(self.notebook) 

    self.notebook.add(self.length_frame, text = 'Length') 

    #Combobox 
    self.measurements = StringVar() 
    self.Combobox_Length_Left = ttk.Combobox(self.length_frame, textvariable = self.measurements, values = ('mm', 'cm', 'inch', 'ft', 'yd', 'm', 'km', 'mi'), width = 10,state=['readonly']) 
    self.Combobox_Length_Left.current(5) 
    self.measurements.trace('w', self.update_length) 
    self.Combobox_Length_Left.grid(row = 2, column = 0, padx = 5, pady = 5, sticky = E) 

    self.measurements1 = StringVar() 
    self.Combobox_Length_Right = ttk.Combobox(self.length_frame, textvariable = self.measurements1, value = ('mm', 'cm', 'inch', 'ft', 'yd', 'm', 'km', 'mi'), width = 10,state = ['readonly']) 
    self.Combobox_Length_Right.current(5) 
    self.measurements1.trace('w', self.update_length) 
    self.Combobox_Length_Right.grid(row = 2, column = 2, padx = 5, pady = 5, sticky = E) 

    #Labels 
    self.Conversion = ttk.Label(self.length_frame, text = 'Convertion:').grid(row = 1, column = 0, padx = 5, pady = 5, sticky = W) 
    self.Label_Blank = ttk.Label(self.length_frame, text = '').grid(row = 1, column = 1, padx = 5, pady = 5, sticky = E) 
    self.Label_To2 = ttk.Label(self.length_frame, text = 'to').grid(row = 2, column = 1, padx = 5, pady = 5, sticky = E) 
    self.Label_To = ttk.Label(self.length_frame, text = 'to').grid(row = 3, column = 1, padx = 5, pady = 5, sticky = E) 

    #Entry Boxes 
    self.Text_Length_Left = StringVar() 
    self.Entry_Length_Left = ttk.Entry(self.length_frame, textvariable = self.Text_Length_Left,width = 13) 
    self.Text_Length_Left.trace('w', self.update_length) 
    self.Entry_Length_Left.grid(row = 3, column = 0, padx = 5, pady = 5) 

    self.Text_Length_Right = StringVar() 
    self.Entry_Length_Right = ttk.Entry(self.length_frame, textvariable = self.Text_Length_Right,width = 13, state='readonly') 
    self.Entry_Length_Right.grid(row = 3, column = 2, padx = 5, pady = 5) 
enter code here 

Answer 1

Statt direkt den eingegebenen Text des Gießens zu schweben, können Sie überprüfen, ob ein Minuszeichen am Anfang des Strings gibt es:

def update_length(self, *args): 

    entered_value = self.Text_Length_Left.get() 

    if entered_value.startswith('-'): 
     sign = -1 
     entered_value = entered_value.replace('-', '', 1) 
    else: 
     sign = 1 

    try: 
     v = float(entered_value) 
    except ValueError: 
     self.Text_Length_Right.set('Invalid input') 
     return 
    m = v * sign * self.as_meter [self.measurements.get()] 
    r = m/self.as_meter[self.measurements1.get()] 
    self.Text_Length_Right.set("{:.5g}".format(r)) 

Sie konnten nichts anderes als Zahlen eingeben, weil self.Text_Length_Left.set('') das Eingabefeld gelöscht hat, als Sie dies getan haben.

Ich hoffe, es wird hilfreich sein.