qt-layouts（qt 布局）

Qt Layout Managers

Layout Hierarchy

Qt lays out widgets using layout objects attached to containers. Never call setGeometry() manually — use layouts.

QWidget (parent)
└── QVBoxLayout (attached via setLayout or constructor arg)
    ├── QLabel
    ├── QHBoxLayout (nested)
    │   ├── QPushButton
    │   └── QPushButton
    └── QTextEdit

Core Layout Types

Basic Usage

from PySide6.QtWidgets import (
    QWidget, QVBoxLayout, QHBoxLayout, QLabel, QPushButton, QSizePolicy
)

class MyWidget(QWidget):
    def __init__(self) -> None:
        super().__init__()
        # Pass widget as parent to layout — attaches layout automatically
        main_layout = QVBoxLayout(self)
        main_layout.setContentsMargins(12, 12, 12, 12)
        main_layout.setSpacing(8)

        label = QLabel("Hello")
        main_layout.addWidget(label)

        # Nested horizontal row
        button_row = QHBoxLayout()
        button_row.addWidget(QPushButton("OK"))
        button_row.addWidget(QPushButton("Cancel"))
        main_layout.addLayout(button_row)

Pass the parent widget to the layout constructor (QVBoxLayout(self)) — this is cleaner than calling self.setLayout(layout) separately, and prevents forgetting to attach the layout.

Stretch and Size Policy

Stretch factors distribute extra space when the window resizes:

layout.addWidget(sidebar, stretch=1)    # gets 1/4 of extra space
layout.addWidget(main_area, stretch=3)  # gets 3/4 of extra space

Size policy controls how individual widgets resize:

# Expand to fill available horizontal space
widget.setSizePolicy(QSizePolicy.Policy.Expanding, QSizePolicy.Policy.Preferred)

# Fixed size — never grows or shrinks
widget.setSizePolicy(QSizePolicy.Policy.Fixed, QSizePolicy.Policy.Fixed)
widget.setFixedSize(200, 40)

Common policies: Fixed, Minimum, Maximum, Preferred, Expanding, MinimumExpanding.

Spacers:

from PySide6.QtWidgets import QSpacerItem, QSizePolicy

# Push buttons to the right
layout.addStretch()                          # flexible spacer
layout.addSpacing(16)                        # fixed-size gap
layout.addItem(QSpacerItem(                  # explicit spacer
    40, 20, QSizePolicy.Policy.Expanding, QSizePolicy.Policy.Minimum
))

QGridLayout

grid = QGridLayout(self)
grid.addWidget(QLabel("Name:"),    0, 0)   # row, col
grid.addWidget(name_edit,          0, 1)
grid.addWidget(QLabel("Email:"),   1, 0)
grid.addWidget(email_edit,         1, 1)
grid.addWidget(submit_btn,         2, 0, 1, 2)  # row, col, rowspan, colspan

# Column stretch — second column takes all extra space
grid.setColumnStretch(0, 0)
grid.setColumnStretch(1, 1)

QFormLayout

Use for settings dialogs and data entry forms — automatically handles label alignment:

from PySide6.QtWidgets import QFormLayout

form = QFormLayout(self)
form.addRow("Username:", QLineEdit())
form.addRow("Password:", QLineEdit())
form.addRow("", QPushButton("Login"))   # empty label for button row

QSplitter

from PySide6.QtWidgets import QSplitter
from PySide6.QtCore import Qt

splitter = QSplitter(Qt.Orientation.Horizontal)
splitter.addWidget(sidebar)
splitter.addWidget(main_content)
splitter.setSizes([200, 600])           # initial pixel widths
splitter.setStretchFactor(0, 0)         # sidebar: don't stretch
splitter.setStretchFactor(1, 1)         # main: takes all extra space

# Persist splitter state
settings.setValue("splitter", splitter.saveState())
splitter.restoreState(settings.value("splitter"))

Centering a Widget in Its Parent

# Via layout
layout = QVBoxLayout(self)
layout.addStretch()
layout.addWidget(target_widget, alignment=Qt.AlignmentFlag.AlignHCenter)
layout.addStretch()

Debugging Layout Issues

Widget appears but is zero-size:

• Set a size hint: widget.setMinimumSize(100, 40) or override sizeHint()
• Check that the parent layout is actually attached (self.layout() returns non-None)

Widget not visible at all:

• Confirm show() was called (or parent is visible)
• Check isHidden() and isVisible()
• Ensure no setFixedSize(0, 0) or zero margins collapsing the widget

Layout ignoring size changes:

• Call layout.invalidate() after programmatic geometry changes
• Verify size policy is not Fixed when you want expansion

Margins and spacing defaults:

• Default content margins: 9px on all sides (varies by style)
• Reset to zero: layout.setContentsMargins(0, 0, 0, 0) and layout.setSpacing(0)