'clicked' parameter in QPushButton's constructor

I can connect slot to QPushButton's clicked signal in its constructor this way 
button = QPushButton("Click me",clicked=say_hello)
It worked ! Test example as following:


import sys
from PyQt5.QtWidgets import *
def say_hello():
    print("Button clicked, Hello!")
# Create the Qt Application
app = QApplication(sys.argv)
# Create a button, connect it and show it
button = QPushButton("Click me",clicked=say_hello)
# button = QToolButton("Click me", clicked=say_hello)
# button.clicked.connect(say_hello)
button.show()
# Run the main Qt loop
app.exec_()


However , there seems no parameter named 'clicked' in QPushButton's constructor as described in the doc  , what's the working mechanism behind ? How do I know which parameters I could pass to the constructor?

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